Fluentcms-button

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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 1.7 / 1.8 / 1.9 support

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* Fix regenerating migrations on Python 3 due to str/bytes differences.
* Fix missing migration for non-null fields.

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* Change the center markup code to ``<p class="text-center btn-center-wrapper">``.

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* Fix 500 error, added missing migration that removed the ``block`` field.

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* Add alignment option