Sort runs by start_date

[mbertrand]

What are the relevant tickets?

https://github.com/mitodl/hq/issues/6074

Description (What does it do?)

Sorts runs by start_date (and then enrollment_start, id if prior attribute is null or the same) across several API's (LearningResourceViewSet, ResourceListItemsViewSet)

How can this be tested?

• On the main branch, run ./manage.py backpopulate_mitxonline_data if you haven't already, and wait for the search upsert tasks to finish.

• Switch to this branch.

• Find the id of the resource with readable_id=course-v1:MITxT+8.01.4x

• Go to http://open.odl.local:8063/api/v1/learning_resources/<course_id>/ - the course should have multiple published runs, check that they are in order by start_date

• Find the id of the program with readable_id=program-v1:MITxT+8.01x

• Go to http://open.odl.local:8063/api/v1/learning_resources/<program_id>/items/ - search for the runs of course-v1:MITxT+8.01.4x and verify they are still sorted by start_date

• Go to http://open.odl.local:8063/api/v1/learning_resources_search/?id=<course_id> - the runs probably won't be in the correct order.

• Run ./manage.py backpopulate_mitxonline_data again. After search updates are done, check the url above again, this time the runs should be in the correct order.

[abeglova]

The sort shows oldest runs first. Since newer runs are most likely to be relevant i think it makes more sense to sort the runs from newest to oldest instead