Schema additions and model/data API for Apps

refs #2102 
- Adds new tables for app settings and app fields
- Renamed `owner` to `app_id` to properly reference apps

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 test 003 schema (Apps)

Import tests for AppSettings and AppFields turned out to be more difficult since adding app_settings requires apps to already exist (same for app_fields).