Cleanup new branch/merge request form in issues
What does this MR do?
Cleanup and fix styling of new branch/merge request form in issues.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- Tests added for this feature/bug
- Has been reviewed by UX
- Has been reviewed by Frontend
- Internationalization required/considered
What are the relevant issue numbers?
fixes #41938 (closed)