GUI to manage users

This is a GUI interface to manage user accounts. Use /usermgr command to open it.

The interface is mostly self-documentated, so no worry.

Note that only players with basic_privs can manage all accounts, otherwise they could only manage their own.

WARNING! On large servers (>~100 players), this may cause huge lag. Please test it on local world copies first. Please report the test results to me, as I've never tested such case.