make media_manager a property of MediaEntry in mixin.py

In all cases where get_media_manager(_media_type_as_string) was called in
our code base we ultimately passed in a "MediaEntry().media_type" to get
the matching MEDIA_MANAGER. It so makes sense to make this a function of
the MediaEntry rather than a global function in mediagoblin.media_types and
passing around media_entry.media_type as arguments all the time.

It saves a few import statements and arguments. I also made it so the
Media_manager property is cached for subsequent calls, although I am not too
sure that this is needed (there are other cases for which this would make
more sense)

Also add a get_media_manager test to the media submission tests. It submits
an image and checks that both media.media_type and media.media_manager
return the right thing. Not sure if these tests could not be merged with an
existing submission test, but it can't hurt to have things explicit.

TODO: Right now we iterate through all existing media_managers to find the
right one based on the string of its module name. This should be made a simple
dict lookup to avoid all the extra work.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

mediagoblin/processing/task.py

@@ -20,7 +20,6 @@ from celery.task import Task
 
 from mediagoblin import mg_globals as mgg
 from mediagoblin.db.util import ObjectId
-from mediagoblin.media_types import get_media_manager
 from mediagoblin.processing import mark_entry_failed, BaseProcessingFail
 from mediagoblin.tools.processing import json_processing_callback
 
@@ -47,14 +46,12 @@ class ProcessMedia(Task):
 
         # Try to process, and handle expected errors.
         try:
-            manager = get_media_manager(entry.media_type)
-
             entry.state = u'processing'
             entry.save()
 
             _log.debug('Processing {0}'.format(entry))
 
-            manager['processor'](entry)
+            entry.media_manager['processor'](entry)
 
             entry.state = u'processed'
             entry.save()