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