Fixes bug with adding a list of users using add_list_member

Improves upon fix in 003d738b, based on discussion in Github pull
request #1297.

Splits add_list_member() into add_list_member() and add_list_members(),
where the former only takes a single user or email, and the latter only
takes lists of users or emails.

Also fixes a bug in how mailing lists were being calculated after the lottery.

Replaces #1297.
The `old_email == new_email` will no longer work correctly now; it will always be false, and update lists when not necessary.  As long we don't care about making sure that name changes make it into mailman, it would work to simply fix the condition to be something that will work correctly (e.g. `new_user.email == old_user.email and new_user.is_active == old_user.is_active` or some such).