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
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
Create BulkAddBatch and use it for bulk add

This lets us add a second subclass for the ImportBatch which will have
different inputs and behaviour.
BulkAddBatch should create BulkAddBatchEntries
Basic ImportBatch#create_entries
Add ImportBatchRow

This will represent a row in the import CSV/TSV. It will act like a presenter,
encapsulating knowledge of how to translate the row into a ImportBatchEntry.
Add a basic ImportBatchesController
Add basic specs for ImportBatchesController

Also move the shared examples for a user without permission into
AuthenticationControllerHelpers.
ImportBatchesController creates a batch