include webpack commons bundle on all pages; stub webpack_loader in python unittests

include webpack commons bundle on all pages

quotes

homegrown webpack_loader mock

toggle installed webpack_loader in test env file instead

fix test checking context

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

@@ -0,0 +1,16 @@
+"""
+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 ''