Ask maintainer to add a milestone and labels for merged MRs with no milestone or no labels

Since we want to look at some stats like "average count of MR per developer per milestone" (https://gitlab.com/gitlab-org/quality/team-tasks/issues/11), it's important that all the merged MR have a milestone (and labels).

Conditions:

Action:

ping the assignee (which should be the maintainer that merged the MR) and ask them to add a milestone

We could do the same for labels.