Don't hide errors with mappings

We added this behaviour because we thought "if an error (now) has a mapping, it
has been fixed and so is irrelevant". The problem is, it means that the graphs
don't match the tables (because they are built from DailyHitTotals, which are
not filtered).
Make hits_table partials more readable

Also encapsulate some knowledge.
Reduce duplication of "hit is now" code
Indicate errors which have been fixed
Remove unused step
Move http_status out of Mapping

* Into hits_helper
* Still not exactly ideal (really, this is Bouncer's concern), but it's tiny and
  used only here
Merge pull request #299 from alphagov/dont_hide_errors_with_mappings_2

Don't hide errors with mappings