Add hit_count to Mapping

* For precomputation to assist Sort By Hits
Micro-optimise Site#hit_total_count

* Get it in one hit, don't iterate rows
Make spec language consistent

* We only use "should" in assertions
Refactor Import::HitsMappingsRelations

* Instance it so we're not passing around site
* Remove some duplication around IN clauses
Precompute mapping hit counts

* Part of Import::HitsMappingsRelations.refresh!
* Not too onerous a query
Remove Mapping#with_hit_count

* expensive scope
* hit_count now precomputed and available as an attribute on Mapping
Make validation messages on New URL consistent

The form labels this field as 'Redirects to', but the validation messages were
referring to it as 'New URL'. 'Redirects to is not a URL' reads a little
awkwardly, so let's make it consistent with existing validation messages on
bulk add.
Merge pull request #331 from alphagov/make_validation_messages_on_new_url_more_consistent

Make validation messages on New URL consistent
Fix a spec to use an invalid URL

This looks as if it was left over from the switch to use Addressable
everywhere - Addressable can parse '___' but it wasn't in the whitelist,
so validation was failing but not for the reason indicated in the test
description. This one really is unparseable by Addressable.
Add filtering to the whitelist view

- This behaves more like the organisation view.
Add a Cucumber spec to test that only admins can access the whitelist
Highlight the row containing the domain just added to the whitelist
Merge pull request #318 from alphagov/whitelist_view_improvements

Improve the whitelist view
Bump admin gem to fix IE8 rendering

See: https://github.com/alphagov/govuk_admin_template/pull/27
Merge pull request #332 from alphagov/admin-gem-ie8-fix

Bump admin gem to fix IE8 rendering
Merge pull request #327 from alphagov/precompute-mapping-hit-count

Precompute mapping hit count
Eliminate unnecessary break

The text breaks a bit oddly with a global archive and the hard break. Removing so it flows more smoothly.
Add missing space
Move space in span
Merge pull request #333 from rjc123/patch-2

Eliminate unnecessary break