/media_assets: add basic index and show pages.

* Add a basic index page at https://danbooru.donmai.us/media_assets.
* Add a basic show page at https://danbooru.donmai.us/media_assets/1.
* Add ability to search /media_assets.json by metadata. Example:
** https://danbooru.donmai.us/media_assets.json?search[metadata][File:ColorComponents]=3
* Add a "»" link next to the filesize on posts linking to the metadata page.

Known issues:

* Sometimes the MD5 links on the /media_assets page return "That record
  was not found" errors. These are unfinished uploads that haven't been
  made into posts yet.
* No good way to search for custom metadata fields in the search form.
* Design is ugly.

config/routes.rb

@@ -154,7 +154,7 @@ Rails.application.routes.draw do
       get :check, to: redirect {|path_params, req| "/iqdb_queries?#{req.query_string}"}
     end
   end
-  resources :media_assets, only: [:index]
+  resources :media_assets, only: [:index, :show]
   resources :media_metadata, only: [:index]
   resources :mod_actions
   resources :moderation_reports, only: [:new, :create, :index, :show]