Add "Viewed" keyboard shortcut for MR changes

Release notes

Problem to solve

Tedious hovering over the "Viewed" checkbox while looking at large merge requests. Somehow you can't even scroll files with "j" after pressing "Viewed" with a mouse, so everything you can is scroll and check with a mouse.

Intended users

Everybody who uses keyboard shortcuts and review merge requests.

User experience goal

Speeding up the workflow

Proposal

Add some button like "v" to mark the selected file as viewed.

Further details

Permissions and Security

Documentation

Availability & Testing

Available Tier

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

What is the type of buyer?

Is this a cross-stage feature?

Links / references