canvas - separate courses per session

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/7772

Description (What does it do?)

This PR makes it so that the course folder is included in the readable id when creating canvas learning resource objects.

How can this be tested?

1. Checkout this branch
2. restart celery
3. ingest canvas courses via python manage.py backpopulate_canvas_courses
4. once finished, inspect the readable ids of the learning resource objects where etl_source="canvas"

from learning_resources.models import *
list(LearningResource.objects.filter(etl_source="canvas").values_list("readable_id"))

or check via admin: http://open.odl.local:8063/admin/learning_resources/learningresource/?etl_source=canvas
5. note that the readable ids should look like "{folder_name}-{course_id}"

Additional Context

In the initial implementation we expected the course id in the course_settings.xml to be unique which turned out to not be the case between sessions. This pr resolves that issue.

[mbertrand]

New session-specific courses were created, but the 2 older courses from an earlier command run are still present with test_mode=True. I think any courses not found in the archives during a command run should probably be deleted and/or have test_mode set to False (in addition to published=False), with all their contentfiles deleted or unpublished from regular and vector search.

[shanbady]

I added the deletion of stale canvas courses. Retrieving the current course ids required going into the archive so i funnel up the relevant readable ids and at the end of the command do a deletion+deindex. This also only applies when running the management command since the webhook just tells us to add a new entry.