Share model classes between projects

From @ReubenBond:

Currently, each project which needs a representation of an Order has its own representation. This makes refactoring complex and it leads to inconsistencies. Instead, we should have one or more shared projects which define the domain model.

If we see a benefit to having a mapped type in some cases (eg, types used in Blazor view models), we should do so, but we should simplify where we can.