Merge branch '2489-soft-delete-issues' into 'master'

Soft delete issuables

Fixes #2489 

What still needs to happen: research on the indexes, the gem suggests a [lot of changes](https://github.com/rubysherpas/paranoia#about-indexes) though this is probably a good idea to discuss and I'm unsure on the impact of an omnibus upgrade as I suspect creating about 10 new indexes has a large impact on the downtime.

TODO: 
- [x] Also group owners can ***soft*** delete
- [x] Button should be hidden

See merge request !2982

3 files changed, 9 insertions, 2 deletions

diff --git a/app/models/ability.rb b/app/models/ability.rb
index e22da4806e6..40c529e8117 100644
--- a/app/models/ability.rb
+++ b/app/models/ability.rb
@@ -235,7 +235,9 @@ class Ability
         :rename_project,
         :remove_project,
         :archive_project,
-        :remove_fork_project
+        :remove_fork_project,
+        :destroy_merge_request,
+        :destroy_issue
       ]
     end
 
diff --git a/app/models/concerns/internal_id.rb b/app/models/concerns/internal_id.rb
index 821ed54fb98..51288094ef1 100644
--- a/app/models/concerns/internal_id.rb
+++ b/app/models/concerns/internal_id.rb
@@ -7,7 +7,10 @@ module InternalId
   end
 
   def set_iid
-    max_iid = project.send(self.class.name.tableize).maximum(:iid)
+    records = project.send(self.class.name.tableize)
+    records = records.with_deleted if self.paranoid?
+    max_iid = records.maximum(:iid)
+
     self.iid = max_iid.to_i + 1
   end
 
diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index 9ab72652190..476e1ce7af0 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -58,6 +58,8 @@ module Issuable
     attr_mentionable :description, cache: true
     participant :author, :assignee, :notes_with_associations
     strip_attributes :title
+
+    acts_as_paranoid
   end
 
   module ClassMethods