Refactor models

Completed with refactoring of 
- [x]  Model code
- [x] Controller code
- [x] Views
- [x] Mailer class
- [x] Admin Models

Code-Climate Analysis <a href = "https://codeclimate.com/github/rohandaxini/knowbuddy/compare/refactor_models">here</a>

Can you please review and merge?
Thanks.