CI: check if maintainers can write to MR branch

Check if users activated the 'Allow commits from members who can merge to the target branch' option in their merge request.

Rename "wiki" test in .gitlab-ci.yml to "web", because it runs this new check too.

(Re-submit of !211 (closed), but from my user's pmaports.git fork instead of the official repository, so we can see whether it works.)

See comment below for output that shows this works.