feat(mr review): editing comments on MR diffs

This MR allows users to edit comments on MR diff.

Editing

API failure when submitting the changed comment

Somebody changed the comment since we fetched it

The user experience when this happens is far from ideal. We pretty much tell the user to:

1. edit the comment again (so they can see their text that we failed to update)
2. copy that comment to the clipboard
3. make sure the side panel is open
4. call a command to refresh the sidebar to see the latest discussion

The error handling prevents any data loss, but the process is a bad user experience.

This never happened to me when commenting on GitLab and so I'm not worried too much about the UX.

concurrent-edit-error

Related to #317 (closed)