Properly consume the `externalURL` value in the standalone web server

Created by: valerybugakov

Steps to reproduce:

1. set externalURL in the dev-private site-config
2. sg start web-standalone
3. check the value of the window.context.externalURL
4. the value if different from the one set in the local site-config

Expected behavior:

The externalURL value set in the local site-config is propagated correctly to the client running on the standalone web server.

/cc @taylorsperry @jasongornall