Fix bug that stopped validating migrator rules

fixes bug introduced df6f91d39bc890fc59498c2c1af8bdb09d274355
Add migrator support for importing ContentTypes
Change way dependencies are processed

Previously, dependencies were specified in the config for the purpose of preprocessing them by rewriting old pks (from otm1) with new pks (from otm2). Now, dependencies are automatically written on the new model instance. This removes the need for model-instantiating value transformers, specifying dependencies in both 'common_fields' and 'dependencies', and should also speed up imports because we are setting 'model.<dependency>_id = integer' instead of 'model.<dependency> = freshly queried instance'.
Replace print statements with self.stdout.write
Removed unused logger
Don't create new relics for ones that exist
Add migrator support for threaded comments