Validate New URL against whitelist in database

I've cleared the previous conversation as this PR (and the context) has changed significantly.

The 'admin' permission will only be granted to a handful of people - the Transition Tools team.

This is also a step in the direction of making the app more self-service and breaking away from Redirector...

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
```