net-mgmt/netbox: Update to 2.5.6

While I'm here:

• Pet portlint

Notable changes since 2.4.9:

• Switched from Django 1.11 to 2.1
• Dropped support for Python 2
• Modeling of patch panels and cables with introducion of new pass-through port component type
• Removed deprecated User Activity log
• Introduction of further interface types
• Implemented Select2 for select fields in forms
• Many more enhancements/bug fixes, for all changes see:

https://github.com/digitalocean/netbox/blob/v2.5.6/CHANGELOG.md

Approved by: miwi (mentor)
Differential Revision: https://reviews.freebsd.org/D19178