triage: ensure that CI is run for MRs from community

Use a blocking thread to remember the maintainers to run the CI. The
idea is that the maintainers have to run it manually and resolve the
thread.

Due to a limitation in gitlab-triage it has to be done in a weird way:
it is possible to check if the author is member of the project and it is
possible to check if he/she is member of the parent group, but it is not
possible to check if it's member of either one in a single rule. Because
of this a simple rule "author is not member" cannot be created.

To workaround this we create 2 rules to detect if the author IS a member
of the project or the group. If he/she is, we add a label to indicate
it. Then, a 3rd rule checks if this label is missing, and if it's
missing, it means that he/she is not a member.

This issue has been reported to gitlab-triage but it doesn't seem to be
in active maintenaince: https://gitlab.com/gitlab-org/ruby/gems/gitlab-triage/-/issues/362

.triage-policies.yml

@@ -182,3 +182,47 @@ resource_rules:
               Work is necessary to move it forward.
             labels:
               - stale
+
+      - name: Label for MRs authored by core contributors (member_of project)
+        conditions:
+          state: opened
+          forbidden_labels:
+            - core-contributor
+            - community-contributor
+          author_member:
+            source: project
+            condition: member_of
+            source_id: NetworkManager/NetworkManager
+        actions:
+          labels:
+            - core-contributor
+
+      - name: Label for MRs authored by core contributors (member_of group)
+        conditions:
+          state: opened
+          forbidden_labels:
+            - core-contributor
+            - community-contributor
+          author_member:
+            source: group
+            condition: member_of
+            source_id: NetworkManager
+        actions:
+          labels:
+            - core-contributor
+
+      - name: Label for MRs authored by community contributors and ensure CI is run
+        conditions:
+          state: opened
+          forbidden_labels:
+            - core-contributor
+            - community-contributor
+        actions:
+          labels:
+            - community-contributor
+          comment_type: thread
+          comment: >-
+            Atention maintainers: remember to run the CI!!
+            
+            This MR is authored by a contributor from the community and the CI does not
+            run automatically.