Piccolo

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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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* Migrations now directly import ``Column`` classes - this allows users to
create custom ``Column`` subclasses. Migrations previously only worked with
the builtin column types.
* Migrations now detect if the column type has changed, and will try and
convert it automatically.

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The Postgres extensions that ``PostgresEngine`` tries to enable at startup
can now be configured.

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