Preparation for membership changes

- Cleanup capabilities to behave more like a model object 
- Support user/domains endpoint (in preparation for membership) 
- Expose SHOW_DOMAIN and SHOW_APPLICATION_BY_DOMAIN to allow fast access 
- Use .present? instead of .count > 0 (performance) 
- Switch Application#find_by to Application#find_by_user to uncover Mongoid#find