Pull code from `ContentCard` into `ChannelsCard` and code cleanup

[marcellamaki]

Since the content card has been almost entirely replaced in the hybrid learning work, we can remove the file. However, since ChannelCard is dependent on it, this will require updating the Channel card first.

The recommended subpage, and other various ContentCard wrappers including the Carousel and the group grid can also be removed.

[MisRob]

It might make sense to consider this issue in the broader context of #8533