Tango-articles

* Django 2.0 and Python 3.x compatible
* Removed comments dependency
* Fixed get_absolute_url syntax
* Formally require typogrify
* Corrected and updated tests
* Removed permalink references
* Reduced dependency on tango-photos
* Added migrations

* Removed deprecated assignement_tag

* Additional fixes for Django 2.0

* Adopted Django 2.0 URL syntax

* Additional on_delete arguments for 2.0 compatibilty

* Added on_delete argument for 2.0 compatibilty