Adds handlebars_assets gem

* Implementing handlebars_assets gem to help us transition to Handlebars
  templates for our Marionette views.
* Loads handlebars.runtime in application.js manifest.
Implements Handlebars in all existing Marionette views.

* Configures Marionette.Renderer to use HandlebarsTemplates instead
  of JST.
* Registers two helpers to format currency and dates on Handlebars.
* Converts existing template files to Handlebars templates.