Schema additions for apps.

fixes #2102
- Added app_settings, app_fields, app_fields_relations tables
Implements Models & Data API for 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 for Apps
- Added app_fields to default fixtures
Merge pull request #2250 from halfdan/2138-data-apps

Schema additions and model/data API for Apps