Mailinglist memberships do not reflect current committee membership

E.g. www (mailinglist number 1):

START
www
[THALIA] [WWW]
1
0
<emailadressen>
END

This: https://gitlab.science.ru.nl/thalia/concrexit/blob/master/website/mailinglists/models.py#L27 should also is_active() from https://gitlab.science.ru.nl/thalia/concrexit/blob/master/website/activemembers/models.py#L176