Introduce account document to user document

An account document is attached to a user document if an account of the
user is on the server. It may be missing if the user is on a remote server.

src/api/common/signin.ts

@@ -2,7 +2,7 @@ import config from '../../conf';
 
 export default function(res, user, redirect: boolean) {
 	const expires = 1000 * 60 * 60 * 24 * 365; // One Year
-	res.cookie('i', user.token, {
+	res.cookie('i', user.account.token, {
 		path: '/',
 		domain: `.${config.host}`,
 		secure: config.url.substr(0, 5) === 'https',