AddDbContext method for DI

Okay. I think we need to understand the value of the constructor that just takes DbContextOptions and how we expect to to be used in both ASP apps and other kinds of app. I don't think we need to do it before this code gets checked in, but I think we should understand it in general soon since I think it can be quite confusing right now to know what to do if I just want access to the options from inside my context instance.