Piccolo

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Fix for auto migrations when using custom primary keys (thanks to adriangb and
aminalaee for investigating this issue).

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Migrations can now have a description, which is shown when using
``piccolo migrations check``. This makes migrations easier to identify (thanks
to davidolrik for the idea).

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When ``piccolo_admin`` is installed, an ``admin_demo`` script is made available
on the command line, which launches a Piccolo Admin demo.

It wasn't working due to a missing folder, which has now been fixed.

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Fixed a bug with custom forms - under some situations they would fail to
render. Thanks to sinisaos for discovering this issue. See
`PR 208 <https://github.com/piccolo-orm/piccolo_admin/pull/208>`_ for more
info.

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Added an ``all_columns`` method, to make it easier to retrieve all related
columns when doing a join. For example:

.. code-block:: python

await Band.select(Band.name, *Band.manager.all_columns()).first().run()

Changed the instructions for installing additional dependencies, so they're
wrapped in quotes, to make sure it works on ZSH (i.e.
``pip install 'piccolo[postgres]'`` instead of
``pip install piccolo[postgres]``).

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The database drivers are now installed separately. For example:
``pip install piccolo[postgres]`` (courtesy aminalaee).

For some users this might be a **breaking change** - please make sure that for
existing Piccolo projects, you have either ``asyncpg``, or
``piccolo[postgres]`` in your ``requirements.txt`` file.

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