There are lots of spam users in trac

Spammers register spam accounts to try to submit spam. For some reason a lot of this spam doesn't ever actually materialize, but the accounts do. They litter the password file and the session table. They slow down rendering of ticket pages, because each user has to appear in the "assign to" dropdown. They make the user experience cruddy too, by making that dropdown have so many items.

We should periodically clear out users that have never actually taken an action on the site.