Add better support for targets on Activities

This adds betters upport for targets in the content generation
and on the model itself. Adding getters for properties which would
otherwise require looking up e.g. get_author.

mediagoblin/user_pages/views.py

@@ -289,7 +289,7 @@ def media_collect(request, media):
                              % (media.title, collection.title))
     else: # Add item to collection
         add_media_to_collection(collection, media, form.note.data)
-        create_activity("add", media)
+        create_activity("add", media, target=collection)
         messages.add_message(request, messages.SUCCESS,
                              _('"%s" added to collection "%s"')
                              % (media.title, collection.title))