Add concrete through table for m2m fields from device/inventoryitem/virtualmachine to softwareimagefile

[gsnider2195]

Proposed Changes

Follow-up to #5280

• Add concrete through table for the software_image_files field from device
• Make the foreign key to software_image_file protected with on_delete=models.PROTECT
• Intercept the protected error on softwareimagefile delete and change the protectederror so that it raises on the related model instead of the through table model
  • Cannot delete SoftwareImageFile because it has a protected relation to Device
  • NOT Cannot delete SoftwareImageFile because it has a protected relation to DeviceToSoftwareImageFile
  • Should be able to copy existing logic for DeviceTypeToSoftwareImageFile
• Expose the through table in the API
• Add filters for the through table and apply to API view
• Add API test
• Add filter test
• Repeat above for inventoryitem
• Repeat above for virtualmachine