fix: add postgres triggers to remove deleted users from user_links (#12117)

* chore: add database test fixture to insert non-unique linked_ids
* chore: create unit test to exercise failed email change bug
* fix: add postgres triggers to keep user_links clear of deleted users
* Add migrations to prevent deleted users with links
* Force soft delete of users, do not allow un-delete

coderd/database/migrations/000194_trigger_delete_user_user_link.down.sql

@@ -0,0 +1,26 @@
+DROP TRIGGER IF EXISTS trigger_update_users ON users;
+DROP FUNCTION IF EXISTS delete_deleted_user_resources;
+
+DROP TRIGGER IF EXISTS trigger_upsert_user_links ON user_links;
+DROP FUNCTION IF EXISTS insert_user_links_fail_if_user_deleted;
+
+-- Restore the previous trigger
+CREATE FUNCTION delete_deleted_user_api_keys() RETURNS trigger
+	LANGUAGE plpgsql
+AS $$
+DECLARE
+BEGIN
+	IF (NEW.deleted) THEN
+		DELETE FROM api_keys
+		WHERE user_id = OLD.id;
+	END IF;
+	RETURN NEW;
+END;
+$$;
+
+
+CREATE TRIGGER trigger_update_users
+	AFTER INSERT OR UPDATE ON users
+	FOR EACH ROW
+	WHEN (NEW.deleted = true)
+EXECUTE PROCEDURE delete_deleted_user_api_keys();