Allow merging via pull request but disallow pushing directly to branch

Forcing all merges to a certain branch (except for key people) to go via pull requests is currently possible in branch permissions.

Pull Requests give traceability and the option to comment on code being merged in - in addition, developers can use plugins and merge checks to add additional controls and warnings to the Pull Request page.

This means that automated checks can be added as merge checks and the people that can be allowed to merge is greater than those who can push - a pre-receive hook doesn't allow good feedback or warnings to the user (as well as not having the context of the branch being merged in, the author etc), so allowing only a restricted set of users to push but allowing all to merge a pull request is desirable.