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

db/structure.sql

@@ -1918,7 +1918,8 @@ CREATE TABLE public.upload_media_assets (
     media_asset_id bigint,
     status integer DEFAULT 0 NOT NULL,
     source_url character varying DEFAULT ''::character varying NOT NULL,
-    error character varying
+    error character varying,
+    page_url character varying
 );
 
 
@@ -5777,6 +5778,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20220204075610'),
 ('20220207195123'),
 ('20220210171310'),
-('20220210200157');
+('20220210200157'),
+('20220211075129');