protected_branches.md

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.