Add "Collapse All" button to Merge Request Changes

Description

When viewing a Merge Request is would improve UX to allow users to Collapse all changes.

Proposal

Current:

image

Proposed:

image

You could also hide the collapse button when 100% of diffs are already collapsed, and hide the expand button while 100% of diffs are expanded.

Overview and Use Case

When reviewing merge requests there are scenarios where there might be files that might not need review, or are verbose enough that opening one at a time to review is more convenient to parse. Files like:

  • Generated files
  • Minified files
  • Updated Plugins
  • etc

It would be beneficial to the user to have the opposite action to expand all available in one click rather than clicking to collapse every file individually.

This is an easy win that has the potential to assist anyone who might use it to save time or organize their process.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.