Make automatic mailinglist for active members

Ghost User
2017-12-10T21:12:33+01:00

Currently there's a mailinglist for committee members, but it's not direct; it contains manually entered committee addresses. Instead, we should simply create a list that contains all current committee members and members of the current board.

Spin-off from #388

### Desired behaviour

1. send email to activemembers@thalia.nu
2. have it delivered directly to all current committee members
3. ensure that it's only delivered once