/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.

app/controllers/media_assets_controller.rb

@@ -1,8 +1,15 @@
 class MediaAssetsController < ApplicationController
-  respond_to :json, :xml
+  respond_to :html, :json, :xml
 
   def index
-    @media_assets = authorize MediaAsset.visible(CurrentUser.user).paginated_search(params, count_pages: true)
+    @media_assets = authorize MediaAsset.visible(CurrentUser.user).paginated_search(params, count_pages: false)
+    @media_assets = @media_assets.joins(:media_metadata)
     respond_with(@media_assets)
   end
+
+  def show
+    @media_asset = authorize MediaAsset.find(params[:id])
+    @post = Post.find_by_md5(@media_asset.md5)
+    respond_with(@media_asset)
+  end
 end