Improvement: Add favourite feature to course cards on category index pages

[abias]

This is a follow-up to #555.

In #555, the course cards on the category index pages were added.
The design of these course cards was aligned to the course cards on the 'My courses' page.

However, one feature was not adopted: The favourite feature was not added as the necessary data and JS module was not directly at hand.

It would be worthwhile to investigate if this feature can still be added.