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