Session feedback

[rbento1096]

As a user I want to be able to give feedback to a session I have been registered in.

• Vote must have a rating (from 1 to 5) and an optional comment.
• Voting is anonymous
• Users can only vote after the session has ended - check session.endsAt
• Users can only vote once.

To discuss: should we also be able to give feedback on common sessions? (here we wouldn't have to check if user is registered or not).

It would be interesting to allow the admins to download feedback like in #128 but this feature is not needed as we can download everything from the dynamo db table and that CSV is enough for the programme team. This last part is merely a nice to have