Remove the ability to read and write static assets to Old Mongo

[ormsbee]

These are static assets added to Old Mongo courses in the file uploads page. It uses "c4x:"-style identifiers (AssetKeys with deprecated=True). This will involve:

• Modify StaticContentServer middleware to no longer handle asset keys with deprecated=True. Unsupported key types should raise HttpResponseBadRequest.
• Modify AssetManager to remove any support it might have for deprecated keys.
• Fix all the tests that this breaks.