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 the @site's default_host, rather than a random
one from mySQL's broken GROUP BY implementation.
Merge pull request #71 from alphagov/fix-summary-grouping

Fix summary grouping for hosts