Thunderbird autoconfig could deal with mailing list aliases

[jdreed]

This is the second half of #732:
"The downside is that for those few people who have moira list aliases, this will pick up the wrong info (whereas the old Thunderbird config used ATHENA_USER). However, standard troubleshooting techniques will reveal that their username is incorrect, and I bet they don't use Thunderbird on Athena all that often anyway. We could, in theory, do something awesome with Scripts such that the .xml file is actually script that does some cleverness with moira and hesiod."