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.
Changes update mailman in lottery controller

Add all registered students into the program mailing list, even if they
didn't get enrolled into any classes.
Update mailman based on email, not sendto address
Merge pull request #1343 from learning-unlimited/add-list-member-fix-2

Fixes bug with adding a list of users using add_list_member