Comment 4 for bug 237210

Pre-impl with Curtis, who brought up an excellent point. We can greatly
simplify things, and kill two birds with one stone, if we don't try to
automate the purging on the Mailman side. Thus the implementation proceeds
like so:

- provide a script that the LOSAs can run on Mailman to purge the mailing list
  artifacts

- provide a button on the team's Manage mailing list page (accessible to LP
  admins and mailing list experts) that allows them to Purge the mailing
  list. This latter button deletes[*] all traces of the list on the LP side,
  but doesn't care at all about coordinating with Mailman. Thus, as far as LP
  is concerned, the merge operation can occur.

[*] I think we may want to add a new PURGED state instead of deleting the
row. The reason is that there may be MessageApproval entries pointing to this
mailing list. A person's standing depends on these, so we'd like not to
affect standing when a list is purged.

The other bird this kills is bug 255845, by allowing a re-request for any
mailing list in the PURGED state.