Moodle: Ensure that H5Ps are part of Course backups

1. Review, test and merge: https://github.com/h5p/h5p-moodle-plugin/pull/131
2. Ensure that this is the correct way of handling files when doing backup/restore
3. When doing a backup and restore of Moodle Courses, H5P Content should be restored with its related files and working.
4. Import/Export for courses must work as well.