[Bug 781715] Mobile feedback page

I tried to break this up into a few commits

1. The changes to the desktop view to make it expandable. This is a lot of moving code around without changing a lot of logic.
2. Adding the view and template for the feedback page. This is more than just a different template. Why? Because the desktop template does this weird thing using two copies of the form with prefilled data that we inherited from the old Input. I decided to just rewrite the logic and style from scratch. I think it went well.
3. The "Thanks" page that is shown after submitting feedback.

These should probably all rebase into one when this lands, but I hope this makes things a little easier to review.

r? (@rdalal)