Add Student and Professor model instead of user

The database schema has been changed because the previous schema clubbed
the students and professors which made it time-taking to access each
group. The new shema uses something near to UserProfile, by linking to the
user directly with OnetoOneField.

- Necessary changes have been made in all the views to ensure security and
  same user interface as before.
- Next step would be to add feedback for individual professors.