Upgrade Django to 1.8

[berkerpeksag]

TODOs

• Read all 1.8 release notes
• Check all TODO comments in codebase
• Check django-* packages and upgrade or remove them
  - Remove django-jsonfield when we switch to Django 1.9: https://docs.djangoproject.com/en/1.9/ref/contrib/postgres/fields/#jsonfield
• https://bitbucket.org/schinckel/django-jsonfield/commits/cb01119557930aaa3a7ac1776fb2453cd4b4190c
• WARNINGS: events.Event.categories: (fields.W340) null has no effect on ManyToManyField.
• Switch to use DurationField: https://bitbucket.org/schinckel/django-timedelta-field/
• Silence all ./manage.py check --deploy warnings
• https://docs.djangoproject.com/en/1.8/ref/templates/upgrading/
• Split commits into relevant parts