Add possibility to add new branch rules

What does this MR do and why?

Add possibility to add new branch rules

Add dropdown and modal for branch rule creation

Changelog: changed

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Enable add_branch_rule FF
2. Go to any project Settings > Repository
3. Observe branch rule section, and add branch rule button/dropdown

Related to #388128 (closed)