Single file diffs

Closes #14103. Related (not part of this MR): #19553.

This adds a `diff_for_path` action to each place we render diffs (commit, compare, new MR, existing MR) which renders the diff for a single path. The action is always available with the same params as the 'parent' action, to make it simpler to generate the URIs.

If a diff is bigger than 10 KB, it will be collapsed by default and have a data attribute added. You can then click the message or the filename to expand that diff. For expanded files, you can collapse and expand them, but they won't make any AJAX requests.

![Expand_and_collapse_diffs](/uploads/a4072029085082b85c47006f67ac531c/Expand_and_collapse_diffs.gif)

See merge request !4990

Use Gitlab-Workhorse-Send-Data to send entry

## What does this MR do?

Use Gitlab-Workhorse-Send-Data to send entry:

Closes #19224, Closes #19128

Also requires this MR to work:
https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/53

## Are there points in the code the reviewer needs to double check?

Do we have a test for this?

## Why was this MR needed?

This way gitlab-workhorse does not have to call any API.

See merge request !5094

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5094#note_12987385

With the option to expand and collapse individual diffs, these aren't
needed any more.

This reverts commit bf2a86b73cce332ff8f4392ffc8df501193f32ec.

This reverts commit 68155ee73b549a4f79744bb325542c29d45c71ea, reversing
changes made to 7ebd011ed1de7aee706f07a53c63c90f1c8aa5d4.

# Conflicts:
#	app/helpers/notes_helper.rb
#	app/views/projects/diffs/_line.html.haml
#	app/views/projects/diffs/_parallel_view.html.haml
#	app/views/projects/diffs/_text_file.html.haml
#	features/steps/shared/diff_note.rb

Added blank state to issues

## What does this MR do?

Adds new blank state to issues when no issues exist.

Part of #18519 

## Screenshots (if relevant)

![Screen_Shot_2016-06-24_at_16.37.12](/uploads/0c3f385615b29216ef1137bd6fac06af/Screen_Shot_2016-06-24_at_16.37.12.png)

See merge request !4908

New diff notes

Fixes #12732, #14731, #19375, #14783 

Builds on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4110

To do:
- [x] Get it mostly working
- [x] Validate position validity
- [x] Fix: Don’t link to `#`
- [x] Fix: Base ref can be `nil`, potentially, when the MR has an oprhan source branch => Yep, doesn’t work. We need to store a `start_id`
- [x] Optimize: Fewer duplicate `git diff` compares
- [x] Optimize: Pass paths to `PositionTracer#diff` for faster diffs
- [x] Refactor: Use `head_id` in `MergeRequest`/`MergeRequestDiff` instead of `source_sha`
- [x] Refactor: Convert existing array-based diff refs to the DiffRefs model
- [x] Tweak: Use `note_type` in `Autosave` key
- [x] Tweak: Remove `line_code: note.line_code` from `link_to_reply_discussion`
- [x] Update: `SentNotifications` and reply-by-email receiver
- [x] Update: MR diff notification email
- [x] Update: API (MR, Commit note creation and entity)
- [x] Update: GitHub importer
- [x] Address any other TODO comments
- [x] Fix: Suppress "edited 4 minutes ago"
- [x] Write tests
  - [x] `LineMapper`
  - [x] `PositionTracer`
  - [x] `Position`
  - [x] `DiffPositionUpdateService`
  - [x] `DiffNote`
  - [x] `MergeRequests::RefreshService` / `MergeRequest#update_diff_notes_positions`
- [x] Make sure commits with diff notes don't get cleaned up, since this would prevent the diff notes from being rendered (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5062)

Future improvements:
- Display unresolved comments on files outside the diff, if the comment was added when that file _was_ part of the diff
- Allow commenting on sections between hunks, when expanding the diff using `...`
  - (We'd need to generate line code based on Position if we have it, even if it falls outside bounds of diff)
- `diff_hunk` on diff note API entity
- Show diff hunk in notification email
- Resolved line notes would have a boolean, and be inactive through `notes.any? { !active? || resolved? }`
- Multi line notes would store a number of positions, and do the right thing (™) in grouping and then rendering if the first item is multiline? => true
- Image diff notes could store x,y,width,height instead of old_line,new_line for similar grouping. Does it need a reference to say if it's on old or new? These can't have line_codes, clearly. Rendering would be interesting.
- Show commit line comments in the MR diff
- Comment on specific selected words
- Comment on file header
- Unfold top of discussion diff note
- New diff notes API for commits and MRs

/cc @rspeicher

See merge request !4101