Resolve "Mailing lists api fails without boards"

Closes #842 (closed)

Previous behaviour

Steps to reproduce:

1. Start a local server with no boards (thalia.nu has the same problem, but it already has boards so it can't be reproduced there).
2. Go to /api/v1/mailinglists/

New behaviour

Steps to validate that it works: (Same as above)

1. Start a local server with no boards (thalia.nu has the same problem, but it already has boards so it can't be reproduced there).
2. Go to /api/v1/mailinglists/