Sort run prices on save; make learning resource prices equal "next run" prices

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/4585

Description (What does it do?)

• Modifies the calculation of LearningResource.prices to return the prices of the resource's next run (if any).
• Adds a line to the etl.loaders.load_run function to sort the list of prices before saving.

How can this be tested?

LearningResourceRun.prices sort

• On the main branch, if you haven't already, run backpopulate_mit_edx_data (you will need the EDX_API_ settings from heroku). Check that the following returns some unsorted run prices:

  from learning_resources.models import LearningResourceRun
  for run in LearningResourceRun.objects.filter(published=True, prices__isnull=False):
      if run.prices != sorted(run.prices):
          print(run.id, run.learning_resource.etl_source, run.learning_resource.resource_type, run.prices)

• Switch to this branch, run backpopulate_mit_edx_data again. This time, if you run the above code, no runs should be returned

LearningResource.prices

Go to http://localhost:8063/api/v1/courses and check that the resource prices field is the same as for the next run of that resource.

Additional Context

This PR won't update existing runs with unsorted price, but the ETL pipelines that will do so run every day.

[jkachel]

LGTM 👍