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`.

app/controllers/wiki_pages_controller.rb

@@ -30,6 +30,15 @@ class WikiPagesController < ApplicationController
   end
 
   def show
+    if params[:format].present?
+      request.format = params[:format]
+    elsif params[:id].ends_with?(".html", ".json", ".xml")
+      request.format = params[:id].split(".").last
+      params[:id].delete_suffix!(".#{request.format.symbol}")
+    else
+      request.format = "html"
+    end
+
     @wiki_page, found_by = WikiPage.find_by_id_or_title(params[:id])
 
     if request.format.html? && @wiki_page.blank? && found_by == :title