Polsu's Skin render system

Use Polsu's new skin render system for the player heads, to ensure the overlay doesn't depend on any external servivces anymore.

https://api.polsu.xyz/#tag/Polsu/paths/~1polsu~1minecraft~1player~1skin~1head2d~1%7Buuid%7D/get

I might create another internal endpoint which only requires one request instead of two right now.