Add reviewers to MR form

Issue: #216054 (closed)

What does this MR do?

MR	Changes
!40488 (merged)	Add reviewers to MR form (BE)
👉 this MR	Add reviewers to MR form (FE)
!40424 (merged)	Part 1: Add reviewers to MR widget -- Setup (HAML + JS)
!40926 (merged)	Part 2: Add reviewers to MR widget -- Vue related (mount, components, spec)

Testing Case

This feature is behind a feature flag, to turn on:

rails c

Feature.enable(:merge_request_reviewers)

1. Go to a Merge Request
2. Click the Edit button

• There is a reviewers dropdown

4. Select a reviewer and click "Save". This will redirect you to the MR page
5. Click the "Edit" button again to check the reviewers are saved.

• The saved reviewer(s) will appear

Reviewer	Reviewers

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #216054 (closed)