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.

Replaces #1297.
Forgot to mention this in the commit message, but this also fixes a bug in how mailing lists were being calculated here. Previously, it looks like all students were being added to all lists?