Piccolo-admin

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

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Added custom forms (courtesy sinisaos).

It's very easy to use - just provide a Pydantic model, and a function for
handling posted data. Piccolo Admin will then auto generate all of the UI
necessary.

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Using the ``swagger_ui`` endpoint from Piccolo API for the Swagger docs, so
it works with the CSRF middleware.

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Rewrote `admin_demo` command to expose configuration options on the command
line.

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* Bumped Node dependencies with security warnings.
* Slightly changed light mode styles (blue-grey sidebar instead of grey).
* Fixed the `admin_demo` command which is installed by setup.py - the path was
wrong.

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Modified the UI to support columns with a ``choices`` attribute set. A select
input element is shown.

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