Piccolo

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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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Catching database connection exceptions when starting the default ASGI app
created with ``piccolo asgi new`` - these errors exist if the Postgres
database hasn't been created yet.

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Added a ``help_text`` option to the ``Table`` metaclass. This is used in
Piccolo Admin to show tooltips.

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Added a ``help_text`` option to the ``Column`` constructor. This is used in
Piccolo Admin to show tooltips.

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* Exposing ``index_type`` in the ``Column`` constructor.
* Fixing a typo with ``start_connection_pool` and ``close_connection_pool`` -
thanks to paolodina for finding this.
* Fixing a typo in the ``PostgresEngine`` docs - courtesy of paolodina.

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Fixing a bug with ``SchemaSnapshot`` if column types were changed in migrations
- the snapshot didn't reflect the changes.

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