Add new corporate info page type for staff updates

Tracker: https://www.pivotaltracker.com/story/show/35260037

This is a new public page that departments can create so that staff have somewhere to check when there is an incident, e.g. www.dft.gov.uk/staff-updates. This doesn't need to be navigatable to, instead, staff will simply know the url of the page to check. There will be  separate story to provide a shorted URL to this page, e.g http://gov.uk/hmrc/staff-update