Add ability to scope search to be within a particular section

[isaacwhite]

When browsing within a particular portion of the site, it would be handy if the search could be scoped (by case-by-case user preference) to search within that folder, rather than the entire drive.

This should be pretty easy for us to support at the API level via a simple swap on the parentId, and would be really helpful to improve the quality of search for large Library sites.