[FEATURE] Historical record for completion dates of books

[aaronleopold]

Is your feature request related to a problem? Please describe.

This feature request is a subset of some of the items discussed in #206. Namely:

completed_at should be a list of dates (with at least one)... Mostly so that in the future you can look and be like "i read this on X dates" etc

Describe the solution you'd like

I don't love the idea of storing a list of dates in SQLite, since I either have to stringify it or store as bytes, so perhaps just make a small lookup table for this. Maybe rename read_progress(es) to read_session(s), I know I've been dying to rename it. The idea would then be that completion status could be tracked by something like completion_history or read_history, and read_session would track stuff like the page/epubcfi/etc.

Additional context

The changes to the schema would be somewhat small, but there would be a decent amount of changes in the API and UI required to accommodate this.