[keyserver] Make it possible to run Comm web app without Apache proxying to it

Summary:
To avoid breaking anybody's environment, if the proxy config is left out, we'll continue defaulting to Apache.

This diff makes it easier to test things sometimes, and perhaps somebody will want to run the Web app without a proxy. (That said, we don't currently support HTTPS/TLS without a proxy, so it would probably be a bad idea to do that on production.)

Linear task: https://linear.app/comm/issue/ENG-1221/make-it-possible-to-run-node-without-apache

Test Plan: I tested a local environment proxied over Apache and a local environment without proxying. I did not test a local environment without proxying with HTTPS enabled, as this would require adding support for certificates to the Node server, which is out of scope

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4210