Add site config for Yelp

Ready to roll.

A couple notes:
1. `formOnly` is required since Waltz did not pass Yelp's XSS check otherwise.
2. Waltz does not accommodate a dynamic `formURL`, which in this case handles the redirects after login; so, this config is limited in the sense that all logins are redirected to the homepage instead of the page on which the user selected the log in link.