#4900: New model for cable paths

[jeremystretch]

Closes: #4900

• Introduce a new CablePath model for recording the origin, destination, intermediate nodes, and status of each end-to-end path
• Add a ForeignKey to CablePath on each of the seven path endpoint models
• Drop the old fields for connected_endpoint and connection_status
• Cache peer termination on CableTermination instances
• Remove trace API endpoint from FrontPort and RearPort
• Remove connection_status field from nested API serializers for device components
• Add cable_peer field to API serializer for all CableTermination models
• Overhaul cable tracing logic with the new trace_paths() utility function
• Completely rewrite all cable path tests
• Add the retrace_paths management command (temporarily?)

[jeremystretch]

To avoid jamming any complex logic into schema migrations, I've opted to retain the trace_paths management command and call it within upgrade.sh. This will, by default, automatically create any missing CablePaths. In theory, this should be necessary only when upgrading to v2.10, however I've also included a --force option to force the recalculation of all CablePath objects should the need arise.

I'm still mulling options for conveying the presence of multiple cable paths through a RearPort with multiple FrontPorts mapped to it in the REST API. I've dropped the /trace/ endpoint from pass-through ports for now.