User should be asked for feedback

[alexjamesmalcolm]

This is wicked tricky to do because Heroku resets our embedded h2 database every couple of days. The way that the pantry wants us to implement this feature would mean asking the user for feedback after they've visited the pantry rather than after they've submitted the email. The time difference might be a couple of weeks which we definitely can't guarantee that Heroku won't wipe our dyno. The solution will have to use a persistent database. Heroku doesn't support H2 so I think we'll use postgres.

[alexjamesmalcolm]

Relies on #124 to be done first