filters: Resolve ids of Action Required states

It seems the database may sometimes refuse to use an index when values
used in a filter are not constant (i.e. come from a subquery).
I noticed this behaviour on MySQL when querying for patches in an Action
Required state, with the Archived filter set to Both.
Since the number of Action Required states is pretty much guaranteed
to be small, resolve their ids into a constant list.

Author: fstachura

patchwork/filters.py

@@ -245,9 +245,7 @@ def kwargs(self):
             return {'state': self.state}
 
         return {
-            'state__in': State.objects.filter(action_required=True)
-            .values('pk')
-            .query
+            'state__in': list(State.objects.filter(action_required=True).values_list('id'))
         }
 
     @property