Fix new branch dropdown position and size

What does this MR do?

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1875

• Size dropdown to size to input/button
• Position dropdown below input/button

Before	After

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

Why was this MR needed?

• "The spacing & positioning of the branch dropdown in the new branch form is incorrect", https://gitlab.com/gitlab-org/gitlab-ce/issues/32096

Does this MR meet the acceptance criteria?

• Tests
  • 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 #32096 (closed)