Merge branch 'feature/cross-project-labels' into 'master'

Add support for cross project references for labels

## Summary

Support for cross project references for labels.

## Rationale

1.   Cross project label references are currently not supported in GitLab
1.   `to_reference` method signature in `Label` model breaks the abstraction introduced in `Referable`.

      `concerns/referable.rb:  def to_reference(_from_project = nil)`

      Signatures:

      ```
      label.rb:           def to_reference(format = :id)

      commit_range.rb:    def to_reference(from_project = nil)
      commit.rb:          def to_reference(from_project = nil)
      external_issue.rb:  def to_reference(_from_project = nil)
      group.rb:           def to_reference(_from_project = nil)
      issue.rb:           def to_reference(from_project = nil)
      merge_request.rb:   def to_reference(from_project = nil)
      milestone.rb:       def to_reference(from_project = nil)
      project.rb:         def to_reference(_from_project = nil)
      snippet.rb:         def to_reference(from_project = nil)
      user.rb:            def to_reference(_from_project = nil)
      ```

     This MR suggests using `def to_reference(from_project = nil, format: :id)` which makes use of keyword arguments and preserves abstract interface.

1.   We need support for cross project label references when we want to move issue to another project

     It may happen that issue description, system notes or comments contain reference to label and this reference will be invalid after moving issue to another project and will not be displayed correctly unless we have support for cross project references.

     Merge request that needs this feature: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2831


I think that cross project label references may be useful, (example: `Hey, see our issues for CI in GitLab CE! - gitab-org/gitlab-ce~"CI"`).

cc @JobV @DouweM @rspeicher 

See merge request !2966

2 files changed, 26 insertions, 55 deletions

diff --git a/lib/banzai/filter/abstract_reference_filter.rb b/lib/banzai/filter/abstract_reference_filter.rb
index cdbaecf8d90..34c38913474 100644
--- a/lib/banzai/filter/abstract_reference_filter.rb
+++ b/lib/banzai/filter/abstract_reference_filter.rb
@@ -94,6 +94,8 @@ module Banzai
             object_link_filter(link, object_class.link_reference_pattern, link_text: text)
           end
         end
+
+        doc
       end
 
       # Replace references (like `!123` for merge requests) in text with links
diff --git a/lib/banzai/filter/label_reference_filter.rb b/lib/banzai/filter/label_reference_filter.rb
index 95e7d209119..8147e5ed3c7 100644
--- a/lib/banzai/filter/label_reference_filter.rb
+++ b/lib/banzai/filter/label_reference_filter.rb
@@ -1,78 +1,47 @@
 module Banzai
   module Filter
     # HTML filter that replaces label references with links.
-    class LabelReferenceFilter < ReferenceFilter
-      # Public: Find label references in text
-      #
-      #   LabelReferenceFilter.references_in(text) do |match, id, name|
-      #     "<a href=...>#{Label.find(id)}</a>"
-      #   end
-      #
-      # text - String text to search.
-      #
-      # Yields the String match, an optional Integer label ID, and an optional
-      # String label name.
-      #
-      # Returns a String replaced with the return of the block.
-      def self.references_in(text)
-        text.gsub(Label.reference_pattern) do |match|
-          yield match, $~[:label_id].to_i, $~[:label_name]
-        end
+    class LabelReferenceFilter < AbstractReferenceFilter
+      def self.object_class
+        Label
       end
 
-      def self.referenced_by(node)
-        { label: LazyReference.new(Label, node.attr("data-label")) }
+      def find_object(project, id)
+        project.labels.find(id)
       end
 
-      def call
-        replace_text_nodes_matching(Label.reference_pattern) do |content|
-          label_link_filter(content)
-        end
-
-        replace_link_nodes_with_href(Label.reference_pattern) do |link, text|
-          label_link_filter(link, link_text: text)
+      def self.references_in(text, pattern = Label.reference_pattern)
+        text.gsub(pattern) do |match|
+          yield match, $~[:label_id].to_i, $~[:label_name], $~[:project], $~
         end
       end
 
-      # Replace label references in text with links to the label specified.
-      #
-      # text - String text to replace references in.
-      #
-      # Returns a String with label references replaced with links. All links
-      # have `gfm` and `gfm-label` class names attached for styling.
-      def label_link_filter(text, link_text: nil)
-        project = context[:project]
-
-        self.class.references_in(text) do |match, id, name|
-          params = label_params(id, name)
-
-          if label = project.labels.find_by(params)
-            url = url_for_label(project, label)
-            klass = reference_class(:label)
-            data = data_attribute(
-              original: link_text || match,
-              project: project.id,
-              label: label.id
-            )
+      def references_in(text, pattern = Label.reference_pattern)
+        text.gsub(pattern) do |match|
+          project = project_from_ref($~[:project])
+          params = label_params($~[:label_id].to_i, $~[:label_name])
+          label = project.labels.find_by(params)
 
-            text = link_text || render_colored_label(label)
-
-            %(<a href="#{url}" #{data}
-                 class="#{klass}">#{escape_once(text)}</a>)
+          if label
+            yield match, label.id, $~[:project], $~
           else
             match
           end
         end
       end
 
-      def url_for_label(project, label)
+      def url_for_object(label, project)
         h = Gitlab::Application.routes.url_helpers
-        h.namespace_project_issues_url( project.namespace, project, label_name: label.name,
-                                                                    only_path:  context[:only_path])
+        h.namespace_project_issues_url(project.namespace, project, label_name: label.name,
+                                                                   only_path:  context[:only_path])
       end
 
-      def render_colored_label(label)
-        LabelsHelper.render_colored_label(label)
+      def object_link_text(object, matches)
+        if context[:project] == object.project
+          LabelsHelper.render_colored_label(object)
+        else
+          LabelsHelper.render_colored_cross_project_label(object)
+        end
       end
 
       # Parameters to pass to `Label.find_by` based on the given arguments