Trainer can remove user from their trainee page.

[santosderek]

Objective:

• Trainer must be able to view list of added trainees, and if remove a trainee from this list.

Features:

• [x] Trainer can view list of all added trainees
• [x] Trainer can click to remove a trainee
  • "Are you sure" prompt must be shown before removal
• [x] Trainee is removed from Trainer's Trainee list in database
• [x] Add another route /remove_trainee that uses the current trainer's session['user_id'] value to remove the trainee from the database.
  • Must also add new function to database for trainee removal if not already there.
• [x] Create unit tests within test_flask and test_database to test these functions.