Add subdomains to host:domain.com searches

Currently, searching for host:domain.com will only find submisions from domain.com and www.domain.com.

However, github project pages are located on sudomain.github.io, and there is no way to search all of them.

Implement and use a reversed() functional index on the domain field to searching for subdomains. Use the same query syntax host:github.io (subdomain search is probably the desired behavior in most cases)