Improve the message list test to test both valid and invalid messages
FilteredModelViewSetMixin: allow filtering boolean fields

Boolean model fields are now special cased in the mixin so as to allow
filtering based on a real boolean comparison, not strings.
Allow filtering the list of all messages based on their validity

This is achieved by mixing in the ``FilteredModelViewSetMixin``
to the appropriate ViewSet and letting it filter only the
"valid" field.

Closes #6