net-mgmt/netbox: Update to 2.5.12 and switch to Django 2.2

Upstream decided to switch from Django 2.1 to the 2.2 LTS release to fix a
rather annoying bug that generated intermittent errors when objects were
deleted/manipulated.

Therefore the same procedure must be done in almost the same way as in
r492690 to make NetBox compatible with Django 2.2 in the ports tree.

That means to repocopy the required Django 1.11 ports, assign them to
Django 2.2, limit them to Python 3.5+ and update all related
CONFLICTS_INSTALL entries accordingly:

• www/py-dj22-django-cors-headers [1]
• www/py-dj22-django-debug-toolbar
• www/py-dj22-django-filter
• www/py-dj22-django-js-asset
• www/py-dj22-django-mptt
• www/py-dj22-django-tables2
• www/py-dj22-django-taggit-serializer
• www/py-dj22-django-timezone-field
• www/py-dj22-drf-yasg

[1] www/py-django-cors-headers was repocopied from r500510 to keep on the

2.5.x branch as 3.x introduced some backward incompatibilities that
haven't been fully tested, yet.

Changelog since 2.5.10:

Enhancements:

• Improve natural ordering of device components
• Add support for filtering cables by connected device
• Add decommissioning status for devices

Bug Fixes:

• Fix natural ordering of device components (after it has been improved before)
• Upgrade Django requirement to 2.2 to fix object deletion issue in the changelog middleware
• Preserve multiselect filter values when updating per-page count for list views
• Fix ordering of interface connections list by termination B name/device
• Fix tagged_items count in tags API endpoint
• Disable last_login update on login when maintenance mode is enabled

Reviewed by: koobs, miwi
Differential Revision: https://reviews.freebsd.org/D20103