Ability to make a comment `unresolved` or `needs resolving`

Resources

PM @victorwu | UX @tauriedavis | BE @DouweM | FE @lbennett

Why?

Sometimes I want to make a comment about something which hasn’t changed in the diff but does need a reply.

What?

• Threads for:
  • Issue comments.
  • Commits
  • Snippets
• Resolvable threads (i.e. resolvable discussions) for:
  • Merge request comments

Design

• The new comment button:
• Button behavior: https://gitlab.com/gitlab-org/gitlab-ce/issues/24378#note_24716939.
• The discussion thread looks and behaves exactly the same as the current one in the diff view for resolvable comments there.