Support pass-through proxies with HTTPS_PROXY env var (#1878)

We've simplified; HTTPS_PROXY or https_proxy is used for all requests.

We also require that only our self-signed certificates are used for
secure traffic. That rules out all SSL-terminating MITM proxies, since
we don't trust their root certificate.

Once we're sure that this system works for people, we'll improve config
on MacOS and Windows.

main.js

@@ -93,6 +93,7 @@ function prepareURL(pathSegments) {
       hostname: os.hostname(),
       appInstance: process.env.NODE_APP_INSTANCE,
       polyfillNotifications: polyfillNotifications ? true : undefined, // for stringify()
+      proxyUrl: process.env.HTTPS_PROXY || process.env.https_proxy,
     }
   })
 }