fix(login): also allow login by email in maintenance mode

fixes #6878
yep, but this duplication already exists in various locations and is currently not reusable;

https://github.com/Elgg/Elgg/blob/1.9/mod/web_services/lib/tokens.php#L149
https://github.com/Elgg/Elgg/blob/1.9/actions/login.php#L35
https://github.com/Elgg/Elgg/blob/1.9/mod/uservalidationbyemail/start.php#L169
https://github.com/Elgg/Elgg/blob/1.9/actions/user/requestnewpassword.php#L12