notebooks/<path> directory urls show error instead of redirecting to tree/<path> urls

[andyscho]

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Problem

Paths in notebooks/<path> url routes that are directories and not notebooks will show an error instead of redirecting to tree/<path> urls.

This behavior differs from Classic Notebook (notebook<7) where directory path links under notebooks/<path> simply redirect to a tree/<path> link and display the directory nicely.

Proposed Solution

Paths in notebooks/<path> urls that are directories should redirect nicely to tree/<path> urls.

#7446 provides a solution to this.