ugoiras: add md5 column to pixiv_ugoira_frame_data.

This is necessary so we can associate ugoira frame data with the media
asset instead of with the post.

db/migrate/20211018045429_add_md5_to_pixiv_ugoira_frame_data.rb

@@ -0,0 +1,10 @@
+class AddMd5ToPixivUgoiraFrameData < ActiveRecord::Migration[6.1]
+  def change
+    add_column :pixiv_ugoira_frame_data, :md5, :string, null: true, default: nil
+    add_index :pixiv_ugoira_frame_data, :md5, unique: true
+
+    up_only do
+      execute "UPDATE pixiv_ugoira_frame_data u SET md5 = p.md5 FROM posts p WHERE p.id = u.post_id"
+    end
+  end
+end