Garry's Mod: add Steam Workshop addons

The cleanest way to add GMod content is a Workshop collection — clients auto-download it when they join.

1. Create a collection on the Steam Workshop for Garry’s Mod and note its Collection ID (the number in the URL).
2. Create a Steam Web API key at steamcommunity.com/dev/apikey.
3. On the panel Startup page, set the Workshop Collection ID and Workshop API key fields.
4. Restart — the server downloads the collection, and players auto-download it on connect.