Removed challenge and submission ids from files

- Removed File model and added ChallengeFile and SubmissionFile models. …
- Swagger: FileResource still exists and acts as entity of both ChallengeFile and SubmissionFile.
- Added tables creation and migration scripts.
- Changed getRefIdField (routeHelper.js) method to return 'fileId' for any mdel name that contains 'File' on its name.
- Fixed swaggerTestHelper to allow definitions without a required setting.