Piccolo-admin

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Add back JSON formatting in list view which was removed by accident.

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Nullable number fields now work correctly.

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Fixed a bug with nullable ``Boolean`` columns - filtering wasn't working in the
sidebar.

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Added the ``link_column`` option to ``TableConfig``. By default, the primary key
is used in the list view of Piccolo Admin to link to the edit page. Using
``link_column`` you can specify a different column (for example, if you hid
the primary key using ``visible_columns``). Thanks to sinisaos for helping
with this.

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Tables can now be grouped in the sidebar - this is helpful if you have lots of
tables. To do this, use the ``menu_group`` argument of ``TableConfig``.

Thanks to sinisaos and sumitsharansatsangi for their help with this.

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A fix to make Piccolo Admin work with ``fastapi>=0.89.0``.

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