Improve CSS of Admin UI

[JulianKniephoff]

The admin UI rewrite basically didn't touch the CSS at all, and it's bad. Meanwhile most of our other frontend projects adopted CSS-in-JS solutions, mostly standardizing on Emotion. We should probably follow suit here.

This would also open up avenues of code sharing and reuse via AppKit, probably also by way of #1293.

See also opencast/opencast#6994, though.

[Arnei]

I also thought about this, and concluded that we don't need to do this, at least not yet.

• Emotion will not fix bad CSS. Fixing up our CSS to work with Emotion will make it better, but we could also just fix up our CSS without Emotion :D
• Having more similar tech stacks between our projects would be nice, but I don't think it is that important.
• AppKit provides benefits if you subscribe to its design philosophy. We have not yet decided on how we want to evolve our design, so I think we should wait on that and then reevaluate if AppKit would be beneficial to us here.