uploads: add upload_media_assets.page_url.

This is needed for multi-file uploads. We need to know both the image
url and the page url to set the post's source correctly when converting
an upload media asset into a post.

db/migrate/20220211075129_add_page_url_to_upload_media_assets.rb

@@ -0,0 +1,5 @@
+class AddPageUrlToUploadMediaAssets < ActiveRecord::Migration[7.0]
+  def change
+    add_column :upload_media_assets, :page_url, :string, null: true
+  end
+end