Final port to Python 3 only.

Changelog:

• All __future__ imports have been removed since we are porting everything to Python 3 only
• Django-mailman3 only scrubs the new EmailMessage but the mailbox.Message is still a subclass of old Message API. For this to work with Hyperkitty, we export the email contents and import them using the new constructor. The default class used inside Hyperkitty now is EmailMessage.
• Emails exported from Hyperkitty don't have a cte of quoted-printable anymore and the standard library handles that
• TopThreadsTestCase sometimes fails due to high number of file descriptors open, this happens because we force all async jobs from django-q to run in sync mode (with a configuration flag) and so it reaches the max limit of file descriptors open on some system. Increasing the value of ulimit -n to unlimited usually fixes this. This wasn't changed as no one should be running in sync mode outside of testing.
• There is partial support for Django 2.0, some methods have been changed, for example:
  • All foreignkey fields in models now have a mandatory on_delete argument
  • django.core.urlresolver has been replaced by django.urls at several places
  • Django 2.0 isn't fully supported as one of the dependencies doesn't support it yet.

Thanks to @llohse @thelinuxguy @globin for most of the work on this branch.