Add site config for Box

Closes https://github.com/waltzio/waltz/issues/244

Notes on the `match`:
- Solves the problem that post-login re-directs do not work properly when user logs in via Waltz button from any path deeper than root level on the `www` subdomain. (In other words using `match` in this case was better than `formOnly` since the former allows us to display the Waltz login button on www.box.com [while hiding it on any page deeper than root level on this subdomain] whereas the latter requires user to visit the log in page before Waltz button appears.)
- Facilitates the fact that post log in re-directs *do* work properly when user logs in from any path on `app` subdomain.
- Accommodates Waltz logins via custom subdomians such as https://fox.app.box.com/login