Secure configuration

This is a new approach to the problem of getting different properties for different sub-projects, specifically for obtaining https versions of URLs in Identity (and soon, facia tool).

What we've been trying to work out is how to get state from the current project into common statically. The Play configuration is designed to do exactly that so I propose this, using the Play configuration to set a project-specific flag that can be read by the GuardianConfiguration.

This has also enabled me to roll back the SecureAssetsPlugin change since we now have a flexible solution to secure properties in general.