Puput

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* Add Django 3.2 support. Drop Django <2.2 support.
* Add Wagtail 2.10-2.14 support.
* Add Python 3.9 to travis test matrix.
* Updated some docs.
* Fix circular imports.

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* Add Django 3.0 support. Drop Python 3.5 support.
* Add Wagtail 2.9 support.
* Show authorship on entry page without image.
* Fix duplicate pages on sitemap.

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* Updated blog model to be extensible
* Add possibility of introducing new comment providers

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* Fix `puput_initial_data` command.

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* Add support for Wagtail >2.7.
* Add Python 3.7 and 3.8 to travis test matrix.

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* Add support for Wagtail 2.6.
* Remove Python 2 compatibility code.
* Fix header image in related entries.
* Remove Google Plus share buttons.