Search: Add Cross-Site Search for any JP/WP.com sites

Because instant search gets all data from the API, it should now be possible to get search results from multiple WP sites whether they are on the same multi-site install or not. The Pro Search already supports searching across sites that are on the same multisite install and this is used by WP.com VIP. It is significantly more user friendly to break those boundaries though.

A v1 could just have the configuration happen in code.

v2 may need some configuration in the admin. Maybe in Calypso since that is the only place that really would know about multiple sites.

Other questions:
- What is the maximum number of sites we allow searching across? My first thought is 10. That said, I know we have run queries that search across 400 sites at a time and the new JP Search index should be able to easily search across hundreds. Maybe start with 100?
- Are there filters that won't work right? Post type filtering requires some data that comes from the site? Maybe the user just needs to define the post type on each site when this is an issue? Anything else? What about multiple languages on different sites?

Originally brought up by a user who wanted to show product results from their store on other sites of theirs, but there are a ton of use cases.