Better Payload Definitions

[japfohl]

Not all of the API payload types are well defined. For example: src/api/payloads/UsersPayload.ts provides no information about what the API returns. For all of theses payload types, better definitions should be created in order to make working with the API package a more friendly experience.

[Crisfole]

This is definitely needed. I just stubbed it out so we could keep moving.