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/users/recommendation.ts

@@ -30,7 +30,7 @@ module.exports = (params, me) => new Promise(async (res, rej) => {
 			_id: {
 				$nin: followingIds
 			},
-			last_used_at: {
+			'account.last_used_at': {
 				$gte: new Date(Date.now() - ms('7days'))
 			}
 		}, {