accounts-ui not dependent on global {{currentUser}} handler

Specifically, we don't have an API for defining global handlers right
now (the old Handlebars.registerHelper). We still need to devise
a new pattern for this but in the meanwhile, this is a step towards
getting accounts-ui to work.
enable dynamic templates

specifically, make {{> foo}} where foo is a helper returning
a template object render that template.
use new dynamic template pattern in accounts-ii