Let templates refer to other templates

Implement template management such that a handlebars template may include references to other, nested templates, ad infinitum. This may require use of the server-specific features in gpii.handlebars and not only gpii.handlebars.renderer.standalone.

E.g.

• "Index template" has a reference to the editor template
  • editor template refers to block templates

The aim being to make the markup DRYer