[meta] Merge conflict resolution improvements

1. "Resolve these conflicts" link is too subtle: https://gitlab.com/gitlab-org/gitlab-ce/issues/21135 - will be in 8.16!
2. Merge Conflict Tool merges source and target branches in reverse: https://gitlab.com/gitlab-org/gitlab-ce/issues/21947
3. Show merge conflicts even when the user can't resolve them: https://gitlab.com/gitlab-org/gitlab-ce/issues/22901
4. Show the reason conflicts couldn't be resolved in the UI: https://gitlab.com/gitlab-org/gitlab-ce/issues/26590
5. Enhance merge conflict editor UI: https://gitlab.com/gitlab-org/gitlab-ce/issues/22902
6. It should be obvious that the 'Side-by-side' vs 'Inline' toggle impacts every file on the Merge Conflicts page: https://gitlab.com/gitlab-org/gitlab-ce/issues/23458
7. Allow resolving merge conflicts in binary files: https://gitlab.com/gitlab-org/gitlab-ce/issues/20664
8. Handle rename conflicts in merge requests: https://gitlab.com/gitlab-org/gitlab-ce/issues/20345

3, 4, and 5 are the easiest from the backend perspective. These are roughly in order of importance.