Adding ASSET_URL to env list and proxy configs

[svenvg93]

This PR updates;

• Env list with the ASSET_URL in case when using a reverse proxy
  • listed as a solution here Bug using HTTPS speedtest-tracker#1673
• Add configuration example for when using a Cloudflare tunnel as proxy
• Update the Traefik configuration example with the ASSET_URL`
• Update the Feature list with the latest notification options.

[Phaeton]

It would be nice to see this one merged in, specifically the bit on Traefik needing the ASSET_URL set. I was getting a bunch of insecure browser warnings until I found the undocumented env variable in a few open issues. I found this PR right before I submit a change to the docs myself.