Check for existing session in swid inventory view

Show an error message when viewing the inventory view for a device that
has no sessions.

Fixes tnc-ba/strongTNC#158.
Added missing load statement
Added swid measurement controller
Added software-id field to model and updated swidimport command
Added software_id field to tag model
Added software_id field to swid_tags table
Removed obsolete method
Applied model changes to view
Added todo note
Fixed PEP8 issue
Added nested entity roles to TagSerializer
Enabled filtering on TagSerializer
Implemented API endpoint for adding SWID tags