Fix summary grouping for hosts

When we brought back hit query forms from the Postgres spike
to avoid mySQL sloppiness, we left a GROUP BY host_id in there.
That host ID has also been taken out of the query. We now link
from hits only to `@site.default_host`, rather than a random
one from mySQL's broken GROUP BY implementation.

This addresses duplicates in the hits tables (for, e.g., /sites/ago/hits)