Refs #2170
This removes the circular dependency problem from our models thanks to
https://github.com/tgriesser/bookshelf/issues/181
- add the registry plugin
- switch all models and collections to be registered
- switch relationships to be defined using a string, which calls from the registry
closes#2138
- Adds new models for AppField and AppSetting
- Removed permitted attributes from App model (handled by base)
- Added reference from Post to AppFields
- Added fixture data to DataGenerator
- Added integration tests for Apps, AppSettings, AppFields
- Added import for Apps
- Added app_fields to default fixtures