Adding shortcuts modules

Shortcuts modules were added for the events and superevents apps.
Currently, they just contain 'is_event()' and 'is_superevent()'
methods.

gracedb/events/shortcuts.py

+from .models import Event
+
+def is_event(obj):
+    return isinstance(obj, Event)

gracedb/superevents/shortcuts.py

+from .models import Superevent
+
+def is_superevent(obj):
+    return isinstance(obj, Superevent)