Fluentcms-contactform

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* Added Django 3 and 4 support.
* Dropped Python 2.7 support.
* Dropped Django 1.10 / 1.11 support.

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* Added Django 2.0 support.
* Dropped Django 1.8 / 1.9 support.

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* Added Django 1.11 support.
* Fixed submitting emails in Django 1.10.
* Changed compact form style: removed the ``:`` sign, added "(optional)" to phone field.

**NOTE:** As of Django 1.10, make sure ``{% autoescape off %}`` is used in the ``.txt`` templates.
Previously, this behavior could be controlled from Python code.
That is no longer possible as of Django 1.10.

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* Avoid Django migrations change when changing ``FORM_STYLE_CHOICES``.

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* Fixed Python 3 support.

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* Added ``type="tel"`` for phone number field.
* Fix shortened placeholder text for phone number field.