Prepare for new WP-API URLs

[nylen]

We're in the process of migrating WP-API URLs as follows (ref: pMz3w-5l5-p2)

- public-api.wordpress.com/wp-json/sites/4/wp/v2/types
+ public-api.wordpress.com/wp/v2/sites/4/types

- public-api.wordpress.com/wp-json/sites/4/wpcom/v2/post-counts
+ public-api.wordpress.com/wpcom/v2/sites/4/post-counts

For a short time, we'll need to support both URL patterns. I propose to do this by detecting the presence of a / in the apiNamespace parameter. Further reading on API namespaces: http://v2.wp-api.org/extending/adding/#namespacing

I expect that client code will call API requests as follows:

this.wpcom.req.get( '/sites/%s/post-counts', { apiNamespace: 'wpcom/v2' } );

Then the transport library (such as this one or rest-proxy) will combine the namespace and the request path.

cc @timmyc @retrofox

[retrofox]

🚢