Closes #5337: Add "splice" port type

[JonathonReinhart]

Fixes: #5337

This adds the splice port type.

[JonathonReinhart]

Notes:

• I did not add a changelog entry. The contributing guide didn't mention this, and I don't know into what version this enhancement will go.
• I did not test this
  • In production, I run Netbox via docker. I struggled (for only a few minutes) to run Netbox out of my git repo. I looked at these directions but they apply to a "real" installation, not a development test. When I run python3 manage.py runserver, I get this error, despite putting configuration.py in various places in the tree:

    ImportError: cannot import name 'configuration' from 'netbox' (/home/jreinhart/git_repos/netbox/netbox/netbox/__init__.py)