Piccolo-admin

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* Added page size selector - courtesy of sinisaos.
* Minor fixes

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Added bulk deletion, and a custom widget for `timedelta` - courtesy of
sinisaos.

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Added a CSV export button to the row listing - courtesy of sinisaos.

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* Removed dependency number for ``uvicorn`` and ``Hypercorn`` - only the very
high level API is being used, which is unlikely to change, and was causing
issues for some users when installing via Poetry.
* Bumped node dependencies.

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Fixing packaging issues - add Python 3.8 classifier, and missing index.html
file.

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Subtle UI fixes - page selector, and ``setTimeout`` typo.

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