Puput

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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.

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* Add support for Wagtail 2.4.
* Update Django version to greater than 2.1.6 to fix security issues.
* Updated some docs.
* Fix entry header image.
* Adding EntryAbstract.content_panels.

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* Updated some docs.
* Fixing social share urls to match `entry_url` (Thanks to misraX).
* Django URLS upgrading to 2.0 and Wagtail 2.3 support (Thanks to MiltonLn)
* Adding support for Django 2.1 and Wagtail 2.3 patch versions (Thanks to MiltonLn)
* Arabic translation (Thanks to ahwebd)