Reapply "Switch container factory to webpack"

This reverts commit 18d93b00ba.

openedx/tests/util/webpack_loader/templatetags/webpack_loader.py

@@ -1,16 +0,0 @@
-"""
-This module mocks the external package django-webpack-loader within
-python unittest execution, so python tests have no dependency on
-frontend assets. See LEARNER-1938 for further details.
-"""
-from django import template
-
-register = template.Library()
-
-
-@register.simple_tag
-def render_bundle(bundle_name):
-    """
-    This is the only webpack_loader function we call directly.
-    """
-    return ''