Merge branch '143-admin-py' into 'master'

Resolve "admin.py"

Closes #143

See merge request startinblox/djangoldp-packages/djangoldp!73

djangoldp/admin.py

+from importlib import import_module
+
+from django.conf import settings
 from django.contrib import admin
-from .models import LDPSource
+from .models import LDPSource, Model
+
+for package in settings.DJANGOLDP_PACKAGES:
+    try:
+        import_module('{}.models'.format(package))
+    except ModuleNotFoundError:
+        pass
+
+model_classes = {cls.__name__: cls for cls in Model.__subclasses__()}
+
+for class_name in model_classes:
+    model_class = model_classes[class_name]
+    if not admin.site.is_registered(model_class):
+        admin.site.register(model_class)
 
 admin.site.register(LDPSource)