Create protected branch via merge request
### Description
When creating merge requests it should be possible to create protected branches if you have permission to merge to a named branch. This would allow a workflow where branch creation is subject to peer review. Today this permission is difficult to achieve as issues gitlab-ce#22638 and gitlab-ce!8458 show.
### Proposal
Allow merge requests to target new branches, as long as the user has permission to merge to that branch.
### Links / references
### Documentation blurb
#### Overview
#### Use cases
Branch creation should be reviewed.
### Feature checklist
issue