UI: add required text to top of form (#33699)

[lukastocker]

https://mantis.ilias.de/view.php?id=33699

This fix will not only add changes to the UI Form but also to the Legacy Form as it's still used at multiple positions (e.g. course settings).

[lukastocker]

Hi @Amstutz,

please have a look at this implementation especially the design part.

Regards, Luka

[Amstutz]

looks good, thx a lot!