account page: can now reorder entries under shops sections by drag and drop

database_migrations/000017_account_page_shops_order.down.sql

@@ -0,0 +1 @@
+DROP TABLE accounts_page_platform_order_indexes;

database_migrations/000017_account_page_shops_order.up.sql

@@ -0,0 +1,8 @@
+CREATE TABLE accounts_page_platform_order_indexes (
+	account_id		INTEGER		NOT NULL,
+	platform		platform	NOT NULL,
+	order_index		SMALLINT	NOT NULL	CHECK (order_index >= 0),
+
+	PRIMARY KEY (account_id, platform),
+	UNIQUE (account_id, order_index)
+);