Create a fake MediaEntry.media_data for sql

So that the SQL backend is more useable, let the MediaEntry
have a faked media_data.
It's extremely fake: The returned dict is always a new one.
So any stored info is even lost!

mediagoblin/db/sql/models.py

@@ -167,6 +167,11 @@ class MediaEntry(Base, MediaEntryMixin):
         if media is not None:
             return media.url_for_self(urlgen)
 
+    @property
+    def media_data(self):
+        # TODO: Replace with proper code to read the correct table
+        return {}
+
 
 class MediaFile(Base):
     """