Reorganize how routing deals with FfA.

I didn't like the way this was originally implemented. It split routing
logic into several functions, and it seemed inelegant to me. This change
makes the special case of handling Firefox for Android's old feedback
method relatively self contained in a separate view. Then it collapses
all the routing logic back into a single function.

To handle the CSRF problem, the routing view is now csrf_exempt, and
each individual prodchan view is marked as csrf_project, except for
FfA's specific view.

This commit essentially removes the changes from commit
70bfa7096fd64e6aea6f849d63878b1c8d6c6427 and rewrites them a in
different way.