CI: bring back mr-settings check (MR 2234)

Add the check again that ensures we can write to the target repository.
This is needed for our merge style, we add the MR-ID to the title of the
commit message, and may do small fixups while merging.

.gitlab-ci.yml

@@ -1,5 +1,14 @@
 image: alpine:edge
 
+# The mr-settings check needs to run in a MR specific context. With this block,
+# the whole pipeline runs in that context for MRs. Otherwise we would have two
+# pipelines for MRs.
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_COMMIT_BRANCH == 'master'
+    - if: $CI_COMMIT_BRANCH == 'wip'
+
 before_script:
   - ".ci/note.sh"
   - "echo 'https://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories"
@@ -37,3 +46,13 @@ vermin:
   stage: lint
   script:
     - ".ci/vermin.sh"
+
+mr-settings:
+  stage: lint
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+  before_script:
+    - "apk -q add python3"
+    - "wget -q 'https://gitlab.com/postmarketOS/ci-common/-/raw/master/check_mr_settings.py'"
+  script:
+    - "python3 ./check_mr_settings.py"