formalize advisors

[tpolecat]

This formalizes the Advisor role.

[rossabaker]

Please don't construe this as me wanting to be chair, but since it's vacant (#24):

• a quorum is 5 votes
• the amendment is affirmed with at least 2/3 of voters, maximally 6
• it would be neat if a workflow did this math

[tpolecat]

Make that 5 since I can't approve my own PR, but :+1: