ARCHBOM-1494: Refer to custom attributes, not metrics, especially with edx-django-utils (#25010)

This uses the new names introduced in edx-django-utils
3.8.0 (edx/edx-django-utils#59), which we're already using, as
well as updating a few other locations where we incorrectly refer
to New Relic custom metrics instead of custom attributes.

Includes a couple of unrelated lint fixes in a file I modified.

cms/templates/widgets/user_dropdown.html

@@ -4,7 +4,7 @@
   from django.conf import settings
   from django.urls import reverse
   from django.utils.translation import ugettext as _
-  from edx_django_utils.monitoring import set_custom_metric
+  from edx_django_utils.monitoring import set_custom_attribute
   from student.roles import GlobalStaff
 %>