Fix #4591: Wiki pages with filename-like name are broken by default.

Fix wiki pages like this returning 406 errors:

* https://danbooru.donmai.us/wiki_pages/rnd.jpg

Caused by Rails parsing the .jpg part as a file extension and trying to
return a JPEG in response. This happens deep in Rails' MIME negotiation
code, so it's hard to override. The fix is to pass `format: false` in
the route to disable all special handling of file extensions by Rails,
and then handle it ourselves in the controller. Ugly.

This only affected two tags: `rnd.jpg` and `haru.jpg`.

config/routes.rb

@@ -280,7 +280,7 @@ Rails.application.routes.draw do
   resources :webhooks do
     post :receive, on: :collection
   end
-  resources :wiki_pages, id: /.+?(?=\.json|\.xml|\.html)|.+/ do
+  resources :wiki_pages, id: /.+/, format: false do
     put :revert, on: :member
     get :search, on: :collection
     get :show_or_new, on: :collection