Change: Remove UiKit

We have removed the UiKit library this design system originally was built on.
The necessary style rules for the design system itself and our web repository have been internalized, 
and everything else got dropped to greatly reduce bundle size and build times.

Please note that with this change, we have also dropped and/or refactored a lot of CSS classes and correlated styling.

https://github.com/owncloud/owncloud-design-system/pull/1658
https://github.com/owncloud/owncloud-design-system/issues/538
https://github.com/owncloud/owncloud-design-system/pull/1882
