Improve database switching

[bcanseco]

Right now, the db name is hard coded inside Startup. https://github.com/bcanseco/warbler/blob/2e08aa22428e4f060d998f14febc7752a4f7ced1/src/Warbler/Startup.cs#L51

AC:

• [x] Ability to switch between dev and prod without changing a checked-in file.
  • [ ] Possibly at runtime from the UI?