Use antd Tree in skeleton view

[daniel-wer]

When the skeleton view was built we couldn't use the antd Tree component due to performance reasons, because it was not virtualized. However, with v4 the Tree component became virtualized and we could switch to it now.

Personally, I see the following benefits:

• More uniform design
• https://github.com/frontend-collective/react-sortable-tree appears to be effectively abandoned
• Drag-and-dropping is rather clunky currently (+ has problems in Firefox) and appears to work smoother in antd