Permissions for sober signup types

[tymorrow]

Liefer requested that the SAA be able to control which members have the ability to sign up for certain sober types based on some arbitrary reason. The exact example he gave was that only people with certain training could sign up to be officer.

I have a couple ideas on how to implement this, but it's not high priority because technically the SAA can already control who can/can't sign up for things by simply adding/removing people from a particular sign up.