Track staff onboarding steps in the system as well

[LarsKemmann]

[LarsKemmann]

This was a request that came up in a recent status call.

Personally, I could see this being most useful when staff take on a hosting or family coaching assignment, but maybe it could also enable other policies in the future (like "only these staff roles can perform this step"). I'm not sure how that would work, but it's a thought.

Also, I'm not sure if we should keep the staff roles as a separate part of the app - for the most part they shouldn't mix with the "Volunteers" UI, except in the scenario above where staff are involved in an arrangement.