KCard: Further improve internal implementation of styles

[MisRob]

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Summary

In https://github.com/learningequality/kolibri-design-system/pull/838 that simplified internal implementation of KCard styling and made styles more robust, @AlexVelezLl had some very good ideas on how to improve even more, for example by utilizing gap and flex-direction. I believe these updates would definitely help us to maintain styling of so many layouts better.

See @AlexVelezLl's full suggestion here.

The goal of this issue is to revisit the styles as per Alex's guidance, not only for the snippets he provided, but all KCard styles.

Acceptance criteria

• [ ] There are no visual regressions in any of the layouts