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?
Screenshots (if relevant)
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
Conform by the merge request performance guides
Conform by the style guides
Squashed related commits together
What are the relevant issue numbers?