Whitelist admin UI

For discussion. Needs a little thinking about whether allowing all admins to add to the whitelist is appropriate - perhaps it should be a distinct permission.

The benefit of making it accessible to any admin is that then (in theory) Transition Managers could resolve problems and we can ask them to contact their Transition Manager rather than transition-dev@

Having it in the app has several benefits:
* more self-service-y - @adammaddison, @davidmann or Transition Managers can add missing entries
* instant results - don't need to wait for next deploy of Transition/Redirector
* starts to break the dependence on redirector

![screen shot 2014-05-14 at 14 35 02](https://cloud.githubusercontent.com/assets/93739/2971643/98eff536-db6c-11e3-8819-0f0dc58ad250.png)

The initial setup would be running something like this by hand:
```ruby
lines = %w{
admin.business-events.org.uk
...
www.youtube.com
}
lines = lines.map(&:chomp)
hostnames = lines.reject { |line| line.empty? }
hostnames.uniq!

jamie = User.find_by_email("jamie.cobbett@digital.cabinet-office.gov.uk")
Transition::History.as_a_user(jamie) do
  hostnames.each do |hostname|
    NewURLWhitelistEntry.create!(hostname: hostname)
  end
end
```