net-mgmt/netbox: Update to 2.5.12 and switch to Django 2.2, Create necessary dj22-* ports accordingly

[kai]

Summary

Upstream switched from Django 2.1 to Django 2.2. The reason behind this is that it fixes intermittent errors when objects are deleted/manipulated. (https://github.com/digitalocean/netbox/issues/2621). Django 2.2 is also designed as new LTS release so it makes sense to switch over to that version. (https://www.djangoproject.com/download/#supported-versions)

The whole diff does following actions in short:

• Update www/Makefile
• Repocopy following ports, assign them to Django 2.2 and limit them to Python 3.5 or later:
  • www/py-django-cors-headers -> www/py-dj22-django-cors-headers
  • www/py-django-debug-toolbar -> www/py-dj22-django-debug-toolbar
  • www/py-django-filter -> www/py-dj22-django-filter
  • www/py-django-js-asset -> www/py-dj22-django-js-asset
  • www/py-django-mptt -> www/py-dj22-django-mptt
  • www/py-django-tables2 -> www/py-dj22-django-tables2
  • www/py-django-taggit-serializer -> www/py-dj22-django-taggit-serializer
  • www/py-django-timezone-field -> www/py-dj22-django-timezone-field
  • www/py-drf-yasg -> www/py-dj22-drf-yasg

• Update all related CONFLICTS_INSTALL entries

Test Plan

• poudriere (11.2-RELEASE amd64) for py36 -> OK
• portlint -> OK

[koobs]

Add netbox changes that motivate these changes to the diff, it helps for everyone to have full context, especially if they're unfamiliar.

On deprecat'ing consumer-less django 21 ports, I'd wait and do that in a separate change (if necessary, we can discuss there or offline)

It would be good to clarify in the summary what the current status of django ports are (django, dj22, dj21?) for those that dont know (i dont, but im assuming)

[kai]

Updated the review with a more precise target:

• Update net-mgmt/netbox to 2.5.12
• Include only the required ports that need to be repocopied/modified to match the given requirements

[miwi]

Since we have done that in the same way for all other netbox releases please go for it.