WIP: Merge conflicts editor backend

What does this MR do?

Add a backend to support merge conflicts in an editor. https://gitlab.com/gitlab-org/gitlab-ce/issues/20344#note_14777035 and the commit message for https://gitlab.com/gitlab-org/gitlab-ce/commit/37d81c7fc38af2ac94bf0ae598311640ddc53593 attempt to describe this in words 😉

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

I haven't implemented this:

If you've already clicked to pick ours / theirs on the conflict sections, then edit the file, we resolve those conflicts before showing you the file.

I can do, I'm just wondering how the API should work. Do we always POST to conflict_for_path, and if the request has sections, resolve those first? We also might not want to do this: https://gitlab.com/gitlab-org/gitlab-ce/issues/20344#note_14786550

I'm also still working on adding JSON Schema (https://gitlab.com/gitlab-org/gitlab-ce/issues/20673) validations for these APIs. They should all use basically the same schema, which they didn't before, so that's progress!

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/20344, https://gitlab.com/gitlab-org/gitlab-ce/issues/21288