xmailmaint should know about Mailman lists

[geofft]

(Yes, I realize it doesn't exist yet)

When we get around to having some nice friendly GUI tool to manage mail subscriptions (cf. #119), it would be awesome if it could do the steps needed to figure out what Mailman lists you're on in addition to figuring out what Moira lists you're on. It's possible (find a list you're on, go to its options, and click around), but it's easier to script than to remember/explain.

This may involve figuring out how to use the Firefox cert store from not-Firefox.