Refactor Endjin.FreeAgent.Domain.Timeslip to remove non-API properties

[HowardvanRooijen]

Timeslip contains:

• ProjectEntry
• TaskEntry
• UserEntry

They also have [JsonIgnore] attributes, as they are not part of the underlying HTTP API but are populated by subsequent calls to act as convenience mechanism.

Now that we have the Endjin.FreeAgent.Extensions project, refactor these additions out, to keep the underlying domain type a pure reflection of the FreeAgent API.