www/py-django: Set DJANGO22 option as default

[kai]

Summary

- Switch to Django 2.2 (current LTS release) because Django 1.11 is
  End-of-Life since April.

- Introduce a DJANGO30 option to make use of the Django 3.0 port and set the
  Python version accordingly for each option.

- Keep the DJANGO111 option for now as long as Django 1.11 is still present
  in the Ports tree.

  It's mainly required for www/py-django-mezzanine, www/seahub and their
  dependencies.  There's is work in progress by upstream to make these ports
  compatible for Django 2.2+ .

PR: 245309
Submitted by: wen (via private e-mail, based on)

Test Plan

• poudriere bulk -n a with either DJANGO111/DJANGO22/DJANGO30 option set and BUILD_ALL_PYTHON_FLAVORS=yes -> OK

Note: The variables should be ordered according to the PHB before/after the change.

[sunpoet]

LGTM. Thanks!