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. Because of this, the diff is a little confusing to me. I found that the output of this command was easier to read, as it shows the changes as if the old implementation never happened.

    git diff 70bfa709 25c1d4e6 -- fjord/feedback/views.py

What do you think about this? As far as actual functionality, I think it is roughly the same as the old way, but in my opinion, it is cleaner, and fits better with what I would like the `feedback_router` function to be. If you don't agree, *we don't need to spend a lot of time debating this*, we can just not do this, leave it how it is, and eventually forget about all of this when Firefox for Android gets updated.

r?