Add possibility to change groups in allowed to merge

What does this MR do and why?

Adds possibility to add/remove groups in allowed to merge drawer

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

How to set up and validate locally

1. Enable edit_branch_rules FF
2. Go to Settings > Repository in any project
3. Expand branch rules
4. Add new rule with "Add branch rule" dropdown
5. Go to rule detail page and click on edit button in the "Allowed to merge" setting
6. Drawer should open on the right with possibility to add/remove users

Related to #438968 (closed)