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.
Note that `current_members` was removed; I'm not sure if mailman will whine in a way that matters if existing members are removed; it's certainly inefficient and will make the following print statement incorrect.  That said, this script will likely not be used again, so I don't know that it matters.