Make django-admin optional.

bigbluebutton/django/admin.py

 """Stub for registering models in django-admin for those who want it."""
+from django.conf import settings
 from django.contrib import admin
 
 from .models import APIToken, BigBlueButton, BigBlueButtonGroup, Meeting
 
-
-admin.site.register(APIToken)
-admin.site.register(BigBlueButton)
-admin.site.register(BigBlueButtonGroup)
-admin.site.register(Meeting)
+if "django.contrib.admin" in settings.INSTALLED_APPS:
+    admin.site.register(APIToken)
+    admin.site.register(BigBlueButton)
+    admin.site.register(BigBlueButtonGroup)
+    admin.site.register(Meeting)