First step towards a MediaManager class: Compat one.

To get us moving towards a MediaManager class, the first
idea is to create a class that wraps our current dict based
manager and makes all users happy.

mediagoblin/db/mixin.py

@@ -212,7 +212,7 @@ class MediaEntryMixin(GenerateSlugMixin):
         # than iterating through all media managers.
         for media_type, manager in get_media_managers():
             if media_type == self.media_type:
-                return manager
+                return manager(self)
         # Not found?  Then raise an error
         raise FileTypeNotSupported(
             "MediaManager not in enabled types.  Check media_types in config?")