This is for fixing a regression introduced by:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24969

This fix will allow users who are allowed to push to protected
branches to create protected branches via CLI as well, just
like before. The checks for protected branch creation won't
need to run.

This commit includes changes to add `UserAccess#can_create_branch?`
which will check whether the user is allowed to create a branch even
if it matches a protected branch.

This is used in `Gitlab::Checks::BranchCheck` when the branch name
matches a protected branch.

A `push_to_create_protected_branch` ability in `ProjectPolicy` has been
added to allow Developers and above to create protected branches.