[iOS] Course Image is stretched in Course Details Page

[edschema]

Current Behavior
Some course card images appear stretched on Course Details pages in the iOS app.

Desired Behavior
Course cards should not appear stretched. Resize should not change image aspect ratio: crop is more acceptable than a stretched image.

[edschema]

PR to address the bug: openedx/openedx-app-ios#659

Note: need to check if 16:8.5 is intentional, and if not, fix.