auto-install GSI config

[drweissbrot]

We should provide an easy, "one-click" way to save the Game State Integration config file to the CS directory. This would be a nice QoL improvement and avoid some of the confusion of the current manual setup.

We could show a banner on the config page, prompting the user to do this. This banner should be hidden as soon as we know the GSI connection works, i.e. as soon as we receive data. Should be possible by establishing a websocket connection (to the WS that's already used by the HUD) and waiting for a message from the server; when we receive a message, we know things work and can hide the banner.

Saving the config file shouldn't be too difficult for the standard install path on Windows and Linux, but custom install paths (and Flatpak) might not work.
Asking the user to open their cs2.exe is probably not going to work in the browser, and this functionality should not be exclusive to the Electron wrappers.

This should be disabled in container images #61. The GSI config file needs to be packaged into the prebuilt binaries.

Should be opt-in.
Should support CS:GO too (i.e. install the config file into the csgo directory too, if it exists).