WIP: Thucydides / Marchant ingestion

[jacobwegner]

refs https://github.com/scaife-viewer/beyond-translation-site/issues/194

[jacobwegner]

@jtauber, re #194, I've got two pending PRs:

• https://github.com/scaife-viewer/frontend/pull/86
• https://github.com/scaife-viewer/backend/pull/74

Those changes would:

• Add a distinct "Textual Notes" widget
• Ingest commentary and textual note annotations separately
• Overload reader-commentaries-mode to show textual notes and commentary annotations

It may be better to create a new reader-textual-notes mode rather than overloading, but that would require further refactoring of the frontend widgets / components.