DomainSR: force domain to lowercase

Even though the site generally normalizes domains to lowercase in most
other places, visiting /domain/EXAMPLE.COM (or any other capitalization)
would not show any results, the only working address would be
/domain/example.com. This forces to lowercase, regardless of how the
domain was capitalized in the url visited.
Thing._byID: raise a useful error for non-int ID

Previously, passing a non-integer ID (usually happens with a string)
to _byID would give the "huge thing_id" error, which was confusing.