Initial work on contrib feedback.

* basic FeedbackItem model
* middleware that injects a form into html responses
Added user and request as hidden fields on the form.
Added request data to FeedbackItem.
Styles and javascript for feedback form.
Now using jQuery.
Documentation.
Javascript to AJAX submit the form.
Style improvements.
Form submission handling.
Added an admin for browsing feedback.
A couple useful admin actions.
Missed this migration.
Order feedback items by their timestamp.
Tamper-proofed the form.
Version bump.