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.
Just thought of this while making the comment below: we might want to leave teacher accounts active when deactivating a student/parent/educator account (or vice versa).