Fix small issues in the mailinglists app

Previous behaviour

Steps to reproduce:

1. You could not search for mailinglists
2. Moderated was not clear in the overview
3. The mailinglist sync contained duplicate and empty email addresses
4. List names could be longer than the maximum
5. No dashes in mailinglist names

New behaviour

Steps to validate that it works:

1. You can search for mailinglists again
2. Moderated is now in the overview
3. The mailinglist sync filters out empty and duplicate addresses
4. Maximum of length 60 applied to names
5. Allow dashes in mailinglist names