Student feedback

@todo
 1. send email to students after session asking for feedback and comments
 2. allow submit feedback only once
 3. admin side for feedback review
 4. ability add tutorials list

How it works: each student gets link with unique token for accessing feedback form (tokens are created in Feedback table before/during email sending). When student submits feedback, record with that token will be updated with submitted information.


After running 'rake db:seed' you can access feedback form with this url /feedback/feedback_valid_token