Closes #20564: Many-to-many pass-through port mappings

[jeremystretch]

Closes: #20564

Note: Any changes made for FrontPort or RearPort are also made for their corresponding template models, FrontPortTemplate and RearPortTemplate, unless otherwise notes.

• Introduce the PortMapping model to map port & position pairs between FrontPorts and RearPorts
  • This model establishes a mappings relation on both FrontPort & RearPort
• Add a positions IntegerField on FrontPort to match the existing positions field on RearPort
• Remove the rear_port ForeignKey and rear_port_position IntegerField from FrontPort
• Update REST API serializers
  • Add front_ports on RearPortSerializer
  • Add rear_ports on FrontPortSerializer
• Extend device instantiation logic to replicate all front/rear port mappings from a DeviceType to a newly created Device
  • Introduce the create_port_mappings() function for this purpose
• Update cable tracing logic to respect positions mapped between front & rear ports
• Update FrontPort & RearPort tables
  • Remove rear_port and rear_port_position columns from FrontPortTable
  • Add mappings column to both FrontPortTable and RearPortTable
• Add a front_port_id filter for RearPort and update FrontPort's rear_port_id filter
• Add positions filter for FrontPort and FrontPortTemplate
• GraphQL API
  • Replace rear_port field on FrontPortType with mappings
  • Add mappings to RearPortType
• Update all tests to use PortMapping for front/rear port mapping
• Add debug logging for cable path tracing

TODO

• Add FKs direct to Device/DeviceType on port mappings
• Finish adapting GraphQL API (e.g. add PortMappingTemplateFilter)
• Support mapping of front/rear ports via bulk import
• Support port template mapping via device type import
• Update logic for detecting split paths
• Test setting front/rear mappings via REST API
• Add tests to verify cable paths are updated correctly when front/rear port mappings change
• Clean up debugging code under Cable.from_origin()

[jeremystretch]

The only changes made in this file are to update the test data to use PortMappings. None of the test methods themselves have been significantly modified.

[jeremystretch]

Aside from the incomplete items noted in the TODO list above, I believe this is ready for review.

[arthanson]

This is the Rear Ports detail view, it is showing "Rear Ports" as the included table, should be "Front Ports"

[arthanson]

We May want to move Rear Ports out of the right-hand-side to below all the other columns - with this demo data it is pushing connection and inventory items way down so not very usable.

[arthanson]

On cable trace in the Related Paths I think it would be useful to not link the current path, for example this one is showing the end path "xe-0/0/3" to "xe-0/0/3" but it is still linked in the table and not obvious which one you are on except by looking at the name in the trace box. Having the current one not be linked would just another nice UI indicator of which one.

[arthanson]

Looks good - the main item is fixing the confusing "rear ports" reference, should be "front ports" on the rear ports detail page. Couple other suggested UI tweaks.

[jeremystretch]

On cable trace in the Related Paths I think it would be useful to not link the current path

Agreed, but this improvement would be out of scope for #20564. Do you want to open a separate FR for it?