Do not crash when a MR was made from the same repository, because the
"allow_maintainer_to_push" key does not exist in the GitLab API's
output. Check first if the "source_project_id" is the same as the
"target_project_id", and if it is, do not try to access
"allow_maintainer_to_push" at all.
Check if users activated the 'Allow commits from members who can merge
to the target branch' option in their MRs.
Add the "only" parameter to each job in .gitlab.yml, so the pipeline
can properly run in a "merge request specific context" and give us the
environment variable that contains the MR ID.