Load branches on new merge request page asynchronously

What does this MR do?

Fill the (source and target) branch dropdowns on the new merge request page via Ajax requests.

Why was this MR needed?

For projects with many branches the new merge request page was very slow.

Screenshots

Before	After
slow-branch-picker	fast-branch-picker
slow-branch-picker-private	fast-branch-picker-private

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #37477 (closed)