Merge pull request #16293 from edx/cale/refactor-upgrade-email-resolvers

...core/djangoapps/schedules/management/commands/__init__.py

...oapps/schedules/management/commands/send_course_update.py

...pps/schedules/management/commands/send_recurring_nudge.py

...ps/schedules/management/commands/send_upgrade_reminder.py

...angoapps/schedules/management/commands/tests/test_base.py

...es/management/commands/tests/test_send_recurring_nudge.py

...s/management/commands/tests/test_send_upgrade_reminder.py

openedx/core/djangoapps/schedules/message_types.py

openedx/core/djangoapps/schedules/resolvers.py

openedx/core/djangoapps/schedules/tasks.py

openedx/core/djangoapps/schedules/tests/test_tasks.py

openedx/core/djangoapps/schedules/utils.py

requirements/edx/base.txt