[EE] Add a new `/create_branch` quick action

EE MR for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15176

What does this MR do?

Allows to create a branch using a quick action. The default branch name can be overriden.

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

I've changed how the JS handles quick actions because that was silly and since this action is quite different from the other ones (it doesn't act on the noteable directly).

Screenshots

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15176

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #27801.