377 manifest handler signed indicator

[dpgraham4401]

Description

This PR adds the ability to see whether a handler on a manifest has signed.

Modification

• signed property added to ManifestHandler model and corresponding serializers (ManifestHandlerSerializer and TransporterSerializer)
• signed: string type added to ManifestHandler interface on front end
• use new signed property to indicate whether a handler (generator, transporter, tsdf) has signed a manifests

In addition,

I created a new TrakBaseModel abstract base class for all apps.trak.models classes. This offers us a uniform repr implementation.

Issue ticket number and link

close #377

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Other Stuff