BB work to allow pre-compiling handlebars templates

Projects extending BB should:

1) Create template to override templates/bluebottle/_main_nav.hbs
2) In this template add the code which was inside {% block_verbatim nav_main %}
3) Overriding {% block_verbatim nav_logo %} should now be done in templates/bluebottle/application.hbs (NOTE: we should make this a handlebars partial)