Added users and exercises models
Add user/exercise relationships
Created main models

User
Classroom
Exercise

and useful associations to link them together.

WIP: display classrooms followed by a user in the user view.
Display followed classrooms

using a partial _classroom.
solved db query issues in the user controller.

TODO: complete all views with required infos.
Homepage and nav, step 0

following tutorial, add home page and a few navigation
items.

TODO: develop a clean home page for a signed up user.