Form-designer

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* Started using real JSON fields for the configuration and submitted data
instead of stringifying the JSON explicitly. A side effect of this change may
be that the JSON object key order isn't preserved but it was a bad idea to
rely on this anyway. **Please check carefully if the database migration
preserves past form submissions and form configurations. It worked when I
tested it but you probably want to check twice.**
* Added a date field type to the list to the default fields.
* Rewrote the submissions export to export all available data (also from
removed fields) in order.
* Replaced ``FormSubmission.sorted_data`` etc. with a centralized facility on
the form model which properly handles missing fields, titles, old names etc.
and which doesn't produce stupidly expensive 1+N queries when processing a
list of submissions.

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* Added Django 4.1 to the CI matrix.
* Extended the ``email`` action with the optional ability to add the author of
the sibmission to the Cc: of the sent email.
* Changed mails to prefer the titles of fields instead of their name.
* Included field titles in the XLSX export when possible.
* Removed our usage of ``collections.OrderedDict``, not necessary anymore.
* Changed submissions to save the full URL, not just the path. Cleaned up the
form submissions model while at it.

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* Specified the default ``AutoField`` for the form designer to avoid
migrations.
* Added a reCAPTCHA v3 field to the default field types.
* Added Django 4.0a1 to the CI matrix.
* Disallowed non-sluggy contents on the name field. The old field name is
preserved for exporting form submissions etc. so the change *should* be
backwards compatible but don't rely on it too much.
* Raised the minimum requirements to Python 3.8, Django 3.2.

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* Stopped hardcoding the admin base form class. Overriding the ``form``
attribute on the ``ModelAdmin`` class for forms now works as expected.
* Raised the minimum requirements to Python 3.6, Django 2.2.
* Switched to a declarative setup.
* Switched to GitHub actions.

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* Fixed a typo and changed e-mail to email. Removed the help text from
config options which isn't correct anymore.
* Added code to avoid crashing the admin interface if form submissions
cannot be deserialized and/or rendered.
* Worked around changes in the initialization of change forms in the
administration interface.
* Added Django 3.1 and Python 3.8 to the Travis CI matrix.
* Reordered the configuration options in the administration panel; moved
activation of processors closer to their configuration.
* Fixed a recurring bug where migrations would be created when changing
field types.
* Made the ordering field a bit wider so that the value is still visible
even on small screens.

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* Fixed the config fieldsets code to work when using Django 3.0.
* Added an unit test and docs for the ``"validate"`` config option.
* Passed additional data to the ``"validate"`` config option. Not
accepting arbitrary keyword arguments is now deprecated.
* Changed the forms administration interface to show all form options
from the beginning. This requires a change to the ``form_fields``
configuration option: Instead of a list it has to be a callable
accepting the form instance now.
* Added support for specifying a description for config options.