Function to enforce password change of ISPConfig users

Add a function to enforce that ISPConfig log-in users change their passwords after a certain number of days when they log into ISPConfig.

1. A setting under System > interface > main config where you can set the number of days when a password expires.
2. Add a password expiration date in sys_user table.
3. Add a page that, when a user is logging in and his last password change is the maximum expiration date, he will be forwarded to a page where he must change the password.

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