Add NETWORK_CONTACT_YOUTUBE config for footer link

Add YouTube channel URL configuration option alongside existing
GitHub/Discord/Email contact links. Also crop logo SVG to content
bounds and pass YouTube env var through docker-compose.

docker-compose.yml

@@ -259,6 +259,7 @@ services:
       - NETWORK_CONTACT_EMAIL=${NETWORK_CONTACT_EMAIL:-}
       - NETWORK_CONTACT_DISCORD=${NETWORK_CONTACT_DISCORD:-}
       - NETWORK_CONTACT_GITHUB=${NETWORK_CONTACT_GITHUB:-}
+      - NETWORK_CONTACT_YOUTUBE=${NETWORK_CONTACT_YOUTUBE:-}
       - NETWORK_WELCOME_TEXT=${NETWORK_WELCOME_TEXT:-}
       - CONTENT_HOME=/content
     command: ["web"]