Make resource.prices = most recent published run prices if there is no next run

[mbertrand]

What are the relevant tickets?

Followup to https://github.com/mitodl/hq/issues/4585 and #1085

Description (What does it do?)

If there is no next run, make resource.prices equal to the prices of the run with the most recent start date.

How can this be tested?

Additional Context

• Set required .env variables and run ./manage.py backpopulate_mitxonline_data
• Go to http://localhost:8063/search/?offered_by=mitx&platform=mitxonline&q=%22Product+and+Service+Creation+in+the+Internet+Age%22
• You should see the course "Product and Service Creation in the Internet Age" listed there with a certificate price of $49

[gumaerc]

👍