New file from interface on existing branch

What does this MR do?

Now you can create a new file and select a target_branch != source_branch. If the file that you want to create already exists on the target branch an error message is shown. A glDropdown is now used to select and create a new branch instead of a text field.

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

No

Why was this MR needed?

To Fix the issue: 'New file from interface on existing branch' #24501 (closed)

Screenshots (if relevant)

From this:

To this:

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?

Closes #24501 (closed)