Allow to create new branch and empty WIP merge request from issue page

What does this MR do?

Replaces the "New Branch" button on the issue page with a dropdown with options to create a merge request or just a branch for the issue.

Are there points in the code the reviewer needs to double check?

See if I used the best naming for class, variables, etc.

Why was this MR needed?

To implement !28558

Screenshots

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Implements #28558 (closed)