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.
Going to leave this as-is for now.