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.
Good idea.  I think I'll do the following:
* deactivating a student, parent, or educator account deactivates all student, parent, or educator accounts.
* deactivating a teacher account deactivates all teacher accounts.
* deactivating an admin account doesn't deactivate any other accounts.
I think this still works nicely with the unsubscription stuff below and doesn't end up with inconsistent state, but I will think it through more carefully before I actually do it.