Fix podcast duration frontend display

[mbertrand]

What are the relevant tickets?

Related to https://github.com/mitodl/hq/issues/4432

Description (What does it do?)

Updates the frontend to display podcast episode duration correctly based on the ISO-8601 format.

How can this be tested?

• Run "./manage.py backpopulate_podcast_data"
• Go to http://open.odl.local:8062/search/?resource_category=learning_material&resource_type=podcast_episode, lick on some episodes and make sure the duration appears in a readable format for those episodes that have a non-null/blnak duration.

[shanbady]

looks good 👍