Make automatic mailinglist for active members

One-sentence description

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 (closed)

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