uploads: add upload_media_assets table.

Add a join table that allows multiple media assets (images or videos) to
be attached to uploads. This is for a future ability to upload multiple
files at once.

app/models/upload_media_asset.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class UploadMediaAsset < ApplicationRecord
+  belongs_to :upload
+  belongs_to :media_asset
+end