wiki pages: fix 404s for page titles containing dots.

Bug: links like these returned 404s:

* https://danbooru.donmai.us/wiki_pages/...
* https://danbooru.donmai.us/wiki_pages/.hack//
* https://danbooru.donmai.us/wiki_pages/ssss.gridman

Cause: by default, Rails uses dots in route segments to separate the id
from the format. For example, in /wiki_pages/ssss.gridman, the id is
parsed as "ssss" and the format is "gridman" (as if "gridman" were a
format like "json" or "xml").

We work around this by specifying the regex for the id param manually.
The trick here is to use a non-greedy match-all combined with a positive
lookahead to detect the extension but not include it in the match.

config/routes.rb

@@ -295,14 +295,10 @@ Rails.application.routes.draw do
   resource :user_upgrade, :only => [:new, :create, :show]
   resources :user_feedbacks
   resources :user_name_change_requests, only: [:new, :create, :show, :index]
-  resources :wiki_pages do
-    member do
-      put :revert
-    end
-    collection do
-      get :search
-      get :show_or_new
-    end
+  resources :wiki_pages, id: /.+?(?=\.json|\.xml|\.html)|.+/ do
+    put :revert, on: :member
+    get :search, on: :collection
+    get :show_or_new, on: :collection
   end
   resources :wiki_page_versions, :only => [:index, :show, :diff] do
     collection do