Add create merge checkbox.

What does this MR do?

Adds a checkbox to start a new MR from a branch. Merge https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14586 first. This relies upon it.

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

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/issues/38626

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Fixes #38626