Rewrite MR comments in Vue JS For Maximum Performance

This issue is to track the work that will be done to rewrite the discussions on the MR page in VueJS This is a huge rewrite but also very important to begin the process of improving performance for our MR discussions.

• Create discussions.json endpoint. @jschatz1 !14923 (closed)

• Create initial webpack bundle @fatihacet !14923 (closed)

• Conditionally render old and new discussions with a cookie flag. @fatihacet !14923 (closed)

• Merge issue_comment_form & issue_note_from into one component: https://gitlab.com/gitlab-org/gitlab-ce/issues/38339

• Transform all issues's components into reusable components:


• issue_discussion_locked_widget
 @fatihacet

• issue_discussion
 @fatihacet

• issue_note_actions
 @fatihacet

• issue_note_attachment
 @fatihacet

• issue_note_awards_list
 @fatihacet

• issue_note_body
 @fatihacet

• issue_note_edited_text
 @fatihacet

• issue_note_header
 @fatihacet

• issue_note_signed_out_widget
 @fatihacet

• issue_note.vue
 @fatihacet

• issue_note_app @fatihacet

• issue_placeholder_note
 @filipa https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14964

• issue_placeholder_system_note
 @filipa https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14964

• issue_system_note
 @filipa https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14964

• Move actions, getters and mutation types into vue_shared folder

  • rename issueData to noteableData

• Missing components from MR view:

  • move notes to vue_shared, create issue_notes and include shared notes
  • create mr_notes app
  • Discussion resolutions
  • Threaded discussions
  • Collapsable discussion
  • Image discussions
  • 2up swipe onion skin
  • Commit messages
  • Tons of statuses
  • Resolve in another discussion.
  • Commit IDs with expansion.
  • This merge request is locked. Only project members can comment.
  • Quick actions in textarea.

• Show diff on DiffDiscussions

  • Define & create data structure of MR widget comments & diffs
  • Add diff_note and diff_discussion to discussion response in MR controller
  • diff note header (file name & copy button)
  • diff note with highlighting

cc @victorwu @fatihacet