Add user-generating and password-resetting for contest

The new feature is put under Others tab in the admin panel. Currently it only support generating users with names like %PREFIX%%NUMBER%. Passwords will be randomly set by a new function in function/global.php and generated users are automatically added to the contest.

Unless correct CID inputed, the Generate button will remain disabled. I only did some rough validating due to time limit.

user_exist() and user_create() are modified to accept array as parameter for performance reasons. maybe faster when generating a large number of users..

reset_password() is added.  contains array or arrays like {username,password}.