Refactor SheetOptions and other entities as a EF Core "Owned Type"

[Sebazzz]

Back when this project was ported from EF6 to EF Core, EF Core didn't understand complex types. However, things have changed and it appears EF Core now does understand complex types as owned types.

We should refactor the split-out SheetOptions to this new structure, and investigate whether other entities also need this refactoring.