Enable single file review mode from the MR Changes tab

Problem to solve

The recently shipped feature of reviewing File by file in MRs !35223 (merged) is buried in the User preferences (https://gitlab.com/profile/preferences).

Users should be able to toggle it on and off from the MR page itself.

Intended users

All users of MRs.

User experience goal

Easily toggle on and off the preference.

Proposal

Further details

While we have the feature flag, this needs to be tied to the same feature flag.

Another detail:

• What should we do with the User preference in the settings? Keep these two places always in sync? Or remove it from the user settings?

Documentation

An update to this section is in order: https://docs.gitlab.com/ee/user/project/merge_requests/reviewing_and_managing_merge_requests.html#file-by-file-diff-navigation

Availability & Testing

What does success look like, and how can we measure that?

Links / references

See Epic for related work: &516 (closed)

Release Notes

Using single-file diff mode is useful to improve performance and navigation of large merge requests, however, enabling it previously meant leaving the merge request, going into user preferences, and enabling the option.

This releases introduces the capability to enable single-diff mode right within the merge request view. As you navigate to the "changes" tab of the merge request, simply click the gear icon and check the box labeled Display one file at a time. This will display a single file at a time and enable the prev and next buttons to navigate among files.