Implement folder selection in file source upload modal

[OlegZharkov]

This PR brings lots of features in selecting rendered items on file source upload modal. It's possible now to select folders including all subfolders and corresponding files, also reworked select all button and tracking of selected/unselected and mixed items (directory was partially selected).

Since the logic has a lot of use-cases I included tests.
As a tiny bonus it also tests 'directory mode'.

Example:

P.S. writing tests was fun, while you write them you discover bugs that shouldn't be there and you go fix them

Depends on #12297 and #12307

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• I've included appropriate automated tests.

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[bgruening]

@jennaj this one is for you. You raised this issue somewhere and I do not remember where :)

[OlegZharkov]

thanks for merging it! @jmchilton @mvdbeek