Add current user to gamemode group during installation

[thrashwerk]

Is your feature request related to a problem? Please describe.
I think most users expect gamemode to do things which require their user to be in the gamemode group like changing the CPU governor so when they install gamemode and it doesn't do that they think it doesn't work at all.

Describe the solution you'd like
I think it'd make sense to just add the current user to the gamemode group when gamemode is installed. I don't think it can be done with just systemd-sysusers like with the group creation since it requires concrete values, would need an extra script or something.

Describe alternatives you've considered
Currently you have to manually add your user to the group.