Discovers modules to register from ``settings.INSTALLED_APPS``.

        This makes sure that the appropriate modules get imported to register
        themselves with Horizon.

def _autodiscover(self):
        """Discovers modules to register from ``settings.INSTALLED_APPS``.

        This makes sure that the appropriate modules get imported to register
        themselves with Horizon.
        """
        if not getattr(self, '_registerable_class', None):
            raise ImproperlyConfigured('You must set a '
                                       '"_registerable_class" property '
                                       'in order to use autodiscovery.')
        # Discover both dashboards and panels, in that order
        for mod_name in ('dashboard', 'panel'):
            for app in settings.INSTALLED_APPS:
                mod = import_module(app)
                try:
                    before_import_registry = copy.copy(self._registry)
                    import_module('%s.%s' % (app, mod_name))
                except Exception:
                    self._registry = before_import_registry
                    if module_has_submodule(mod, mod_name):
                        raise