Fix test run failures with double-registration on ICEkitURLField

Use the new `ICEkitURLField.register_model_once` method available in
django-icekit to safely register base events for `ICEkitURLField`
without the risk that they will be re-registered (and therefore fail)
because of the way the unit tests reload this app.

icekit_events/apps.py

@@ -23,5 +23,5 @@ def ready(self):
         autodiscover_modules('event_type_plugins')
 
         from .models import EventBase
-        ICEkitURLField.register_model(
+        ICEkitURLField.register_model_once(
             EventBase, widget=SimpleRawIdWidget(EventBase), title='Event')