Added database cleaning commands

remove_dossiers_without_recommendation and
remove_representatives_without_votes are useful to clean the database
before dumping test fixtures.

representatives_votes/management/commands/remove_dossiers_without_proposal.py

+from representatives.management.remove_command import RemoveCommand
+
+from representatives_votes.models import Dossier
+
+
+class Command(RemoveCommand):
+    manager = Dossier.objects
+    conditions = {'proposals': None}

representatives_votes/management/commands/remove_representatives_without_vote.py

+from representatives.management.remove_command import RemoveCommand
+
+from representatives.models import Representative
+
+
+class Command(RemoveCommand):
+    manager = Representative.objects
+    conditions = {'votes': None}