Remove deactivated accounts from mailman

Everything in here only applies if mailman is in use.

Previously, we did not attempt to remove users from mailman lists if they deactivated their account, nor did we change their email address if it was changed.  Now, we will do so.  In the case where the user has multiple accounts, this gets tricky.  If they change their email, we keep both emails.  If they deactivate their account, we deactivate all other accounts with the same email address.  (The page warns that this will happen.)

This also adds a script to update existing lists.  This is a bit difficult as we don't know how they got the way they are, so the script is a bit cautious.

Unfortunately, this is somewhat untested since I don't have a local mailman install.  I will do some manual testing when I put it on esp.mit.edu, and I think that is the only site that uses mailman.

Fixes #132 and #664.
This has the wrong behavior on some edge cases. Example: parent and student are using the same email, parent changes email, parent's new email is now on all the student's class lists. Other example: person has both a student account and a teacher account with the same email, changes the teacher account's email, is now on announcements@ and all their other student lists with their teacher email.

I'm don't think I'm worried enough about the first case for it to be worth fixing, but we might want to actually consider the second, since if they ask to unsubscribe from announcements@ we'll deactivate their account and now they won't be on teachers@ either.