Add change freeze code

This has rotted since last year, largely due to many new features having been added
in the interim. It has been fixed and improved:

 - The config is now done in the properties file
 - Web based deploys present a modal dialog
 - API calls to disabled stages will fail
 - Continuous deploys for disabled stages will not be triggered (a message is
   displayed to users to make this clear
Merge pull request #139 from guardian/sh-change-freeze

Add change freeze code