support for reverse proxy in front of web-proxy over http

[curiousercreative]

Attempts to make it easier to reverse proxy snikket. Together with this gist, solves snikket-im/snikket-selfhosted#13.

If accepted, next steps to consider:

1. Update documentation for reverse proxy
2. Add "advanced configuration" entry for configuration variable to put web-proxy into reverse proxy mode
3. Add alternative (or override or companion) docker-compose.yml file and link in reverse proxy documentation.
4. Disable reloading when reverse proxied.

[curiousercreative]

How this file was derived:

1. copy https template
2. Remove all SSL bits
3. Use http port

[justinmayer]

Everything I self-host is behind Traefik, an environment that handles TLS termination and that unfortunately Snikket does not currently seem to handle well. Hopefully changes like this PR will make it more feasible to use Snikket with Traefik 👍

[ficematt]

I also have everything behind Traefik. This PR would sure be helpful.