Piccolo-api

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Added a ``swagger_ui`` endpoint which works with Piccolo's ``CSRFMiddleware``.

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Modified the auth middleware to add the Piccolo ``BaseUser`` instance for the
authenticated user to Starlette's ``BaseUser``.

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Add missing `login.html` template.

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Added support for ``choices`` argument in Piccolo ``Column`` instances. The
choices are output in the schema endpoint of ``PiccoloCRUD``.

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Added ``validators`` and ``exclude_secrets`` arguments to ``PiccoloCRUD``.

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Added ``superuser_only`` and ``active_only`` options to ``SessionsAuthBackend``.

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