[Frontend only] Batch comments on merge requests

➡ CE backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22158

What does this MR do?

This MR implements the rest of the frontend work left for #1984 (closed), standing on top of !4213 (merged)

Status of Features

• Start a review by posting the first draft
• Add drafts to existing review
• Show Review bar at the bottom of the screen
  • Make Review Bar display regardless of which tab is selected
  • Show Reviews count
  • Submit Review
  • Dismiss Review
• Show Drafts in the diff line
  • Inline view
  • Parallel view
  • Show the correct content for the Draft
  • Show discussion resolution status on draft
  • Add actions to draft:
    • edit
    • delete (still might need refactoring)
  • Publish draft individually
• Send resolve/unresolve status
• Check that quick actions work at least in Discussion tab
  • They don't. Need to add support to Draft Note vue component to list actions to be taken when published (per mockup)

Status of low-level tasks

• Render drafts using same mechanism as discussions (incremental rendering, after diffs)
• Use old icons for actions (edit / delete) to sync-up visually with the Notes' ones
  • create follow-up issue
• Rewrite mixin (experiment) in app/assets/javascripts/diffs/components/diff_line_note_form.vue to an action in the batchComments store module we kept the mixin. ...

Bugs to fix

• Don't fetch/render drafts when signed out
• Clear form properly after draft submission
• Prevent Batch Comments logic in Issues (start a discussion in an issue is broken)
• Don't display batch comments buttons while editing a current existing note/draft.
• Drafts are shown on same line of all files changed (need to add an extra check)

Nice-to-haves for first release

• Improve "pending" area of Draft (see discussion).
• Improve badge style when “Submit review” button is disabled/loading (screenshot)
• Set drafts on lines upon fetching, as we do with discussions (follow up)

Tests tasks

• Add a complete feature test to ensure with breaks if a conflict is badly resolved between CE-EE

What are the relevant issue numbers?

#1984 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?