Allocate some unowned workers to categories
What does this MR do?
AdminEmailWorker only send repository check emails, so it belongs to source code management. (It also doesn't run on GitLab.com.)
GitlabUsagePingWorker belongs to the collection category.
PruneOldEventsWorker relates to events which are used on the user profile, so I've gone with users. This could also go under audit events, potentially.
AdminEmailsWorker sends an admin-defined email to all users, so I put it under issue tracking as that currently owns notifications (!). (This also doesn't run on GitLab.com.)