Build a new plan for tor-announce and make it happen

We currently use tor-announce only for stable tor releases, and actually since we don't want to announce them until there are TBB packages, lately we've been failing to reliably use it even for that. This is bad for our users.

We should figure out what counts as something all our users should learn about, and consistently mail tor-announce when one of those events happens.

(And I should not be the bottleneck for all of these events.)