Piccolo-admin

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Added a ``visible_filters`` option to ``TableConfig``, allowing the user to
specify which filters are shown in the filter sidebar. This is useful if you
have a lot of columns. Courtesy sinisaos.

Improved the navigation sidebar UI - each section can now be hidden, and the
appearance has been improved when table names are very long. Courtesy
sinisaos.

Added docs for Javascript formatting to help new contributors.

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Added ``TableConfig``, which allows more fine grained control over how the
UI behaves for a given ``Table``. Currently it allows you to specify which
columns are visible on the list page, but more options will be added in the
future. Courtesy sinisaos.

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Fixed bugs with nullable ``ForeignKey`` and ``Timestamp`` columns - the UI
would try sending back an empty string, instead of a ``null`` value. Courtesy
sinisaos.

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JSON values are now displayed in a nicer format in the UI (courtesy sinisaos).

The popup banner displayed at the bottom of the page will now turn red when
showing an error (it was already green in the past). Courtesy sinisaos.

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``FormConfig.endpoint`` now works with async functions.

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Fixing a bug where setting ``FormConfig.description`` to ``None`` caused a
serialisation error.

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