/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.

test/functional/media_assets_controller_test.rb

@@ -3,12 +3,28 @@ require 'test_helper'
 class MediaAssetsControllerTest < ActionDispatch::IntegrationTest
   context "The media assets controller" do
     context "index action" do
+      setup do
+        @media_asset = create(:media_asset)
+      end
+
       should "render" do
-        create(:media_asset)
         get media_assets_path, as: :json
 
         assert_response :success
       end
+
+      should respond_to_search({}).with { @media_asset }
+      should respond_to_search(metadata: { "File:ColorComponents" => 3 }).with { @media_asset }
+      should respond_to_search(metadata: { "File:ColorComponents" => 4 }).with { [] }
+    end
+
+    context "show action" do
+      should "render" do
+        @media_asset = create(:media_asset)
+        get media_asset_path(@media_asset), as: :json
+
+        assert_response :success
+      end
     end
   end
 end