Fixes #89 - Create a connection between Feedback and TaskAttempt

I have changed the model so that Feedback is only associated with TaskAttempt, and User and Task can be found via TaskAttempt.

I also updated the admin, but there are a couple of things I'm having trouble with:

1. How to get the state name, as opposed to the integer value, to appear in the Feedback list.
2. How to get the values from User and Task to appear on the Feedback detail page. I think I need to create a FeedbackModelForm for this, which I did, but then it seems like I need to create fields on the form for each attribute, and I'm not sure if that's the most efficient way to do it.

Also note that the DB changes I made will break any existing feedback data, so it may be best to start with a fresh DB.