Improve activity interface

apps/activity/fixtures/initial.json

@@ -4,6 +4,7 @@
     "pk": 1,
     "fields": {
       "name": "Pot",
+      "manage_entries": true,
       "can_invite": true,
       "guest_entry_fee": 500
     }
@@ -13,6 +14,7 @@
     "pk": 2,
     "fields": {
       "name": "Soir\u00e9e de club",
+      "manage_entries": false,
       "can_invite": false,
       "guest_entry_fee": 0
     }