make media_manager a property of MediaEntry in mixin.py

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>