Add option to set game mode settings per chat command

[dassschaf]

As noted in a TODO in /plugins/admin.js a chat command /admin mode set <S_Setting> <value> is currently broken and needs to be implemented properly.

[GreepTheSheep]

This method works on TM2020 but does not work on MP4 (TM2), maybe recheck our callback

[dassschaf]

I'm not sure if anything works well in TM2 - if you'd want to make it work with TM2 it'd be up to you, it's mainly a controller for TM2020 servers.