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/endpoints/i.ts

@@ -22,7 +22,7 @@ module.exports = (params, user, _, isSecure) => new Promise(async (res, rej) =>
 	// Update lastUsedAt
 	User.update({ _id: user._id }, {
 		$set: {
-			last_used_at: new Date()
+			'account.last_used_at': new Date()
 		}
 	});
 });