Allowing pushing to protected branches enables the Merge button on MRs

Previously (for some value of it) "Allowed to merge" controlled which users received a green Merge button on an MR and "Allowed to push" controlled which users were allowed to push via git push to protected branches.

I think that worked reasonable in the sense that we could not merge mistakenly to gitlab branches that were protected but the torproject-pusher could sync over from git.tpo.

However, no it seems that the "Allowed to push" option is controlling the Merge button, too. Even if no one is allowed to merge on protected branches I get the green Merge button on MRs as long as "Allowed to push" is enabled.

@sysrqb for notice as we talked about that problem earlier.