Add MediaAsset model.

A MediaAsset represents an image or video file uploaded to Danbooru. It
stores the metadata associated with the image or video. This is to work
on decoupling files from posts so that images can be uploaded separately
from posts.

test/functional/media_assets_controller_test.rb

@@ -0,0 +1,14 @@
+require 'test_helper'
+
+class MediaAssetsControllerTest < ActionDispatch::IntegrationTest
+  context "The media assets controller" do
+    context "index action" do
+      should "render" do
+        create(:media_asset)
+        get media_assets_path, as: :json
+
+        assert_response :success
+      end
+    end
+  end
+end