Player avatars

[drweissbrot]

Should be opt-in.
Users should be able to save images in a theme (probably in a player-avatars subdirectoy, filenames matching /STEAM64ID\.(webp|jpe?g|png|avif)/), and a fallback image for players with no avatar image.
These avatars should be displayed in the sidebars.
If possible, there should be an option to enable fetching avatars from Steam, with locally defined avatars taking precedence.

We may also want to show the avatar of the focused player in the focused player section in the middle; this should also be opt-in.
This should be set up so we can add a facecam for the focused player in the future without breaking compatibility.

All of this needs to be explained in the docs, similar to https://github.com/drweissbrot/cs-hud/blob/4c5d98eef34e17f55d42b7895ee3f452ec0f4d5e/docs/team-logos.md, and likely also the FAQ section in the readme.