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/unit/upload_service_test.rb

@@ -937,6 +937,17 @@ class UploadServiceTest < ActiveSupport::TestCase
         assert_equal([], post.errors.full_messages)
         assert_not_nil(post.id)
       end
+
+      should "create a media asset" do
+        post = subject.new({}).create_post_from_upload(@upload)
+
+        assert_not_nil(post.media_asset)
+        assert_equal("12345", post.media_asset.md5)
+        assert_equal("jpg", post.media_asset.file_ext)
+        assert_equal(1000, post.media_asset.file_size)
+        assert_equal(100, post.media_asset.image_width)
+        assert_equal(100, post.media_asset.image_height)
+      end
     end
   end