Comments: Backend / persistent data structure and API

[McManning]

W3C has a standard for annotation data, I think it'll make sense to follow this: https://www.w3.org/TR/annotation-model/

These are some examples of how Adobe uses it for their PDF annotator tool:
https://developer.adobe.com/document-services/docs/overview/pdf-embed-api/howtos_comments/#annotation-data-examples