ThumborMigrationStorage.url() implemented.
try to retrieve the image from Thumbor and return its url if exists, else return
the file-system url.
Add objects to initial_data of thumbor_project.
ThumborStorage.exists(): use the name string to guess if it is a Thumbor location.
ThumborStorage.url(): use a proxy function.
+ get_available_name always return True.
ThumborMigrationStorage.url(): guess which superclass.url() to call according to the name parameter.