Added settings view for target branch rules

What does this MR do and why?

Adds the settings UI including the form for target branch rules.

note the form does not save any records yet

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Enable target_branch_rules_flag feature flag
• Visit /gitlab-org/gitlab-test/-/settings/merge_requests to see the UI

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.