This changes the two interfaces IChangeRequest and
IChangeRequestSchedule to be union types instead of interfaces. It also
extracts the constituents of those union types into proper types
themselves (so that they can be used in function type signatures etc).
It also updates the type names.
This turned out to be more work than I had imagined, but I think the end
result pays off, giving us more type safety and control.
I wanted to use just `ChangeRequest` for the IChangeRequest type, but
that caused issues due to naming collisions with the `ChangeRequest`
component that we have, causing tests to fail. I've named it
`ChangeRequestType` as a potential solution, but suggestions are
welcome.
The relevant changes are in
`frontend/src/component/changeRequest/changeRequest.types.ts`.
Everything else is updated references and some necessary refactoring to
respect the new types.
