promotion/demotion of users to maintainers

[antislice]

No description provided.

[rossettistone]

From discussion between Grayson and me:

"As a repo owner, I want to make sure there are enough maintainers for my project"
"When the project does not have enough maintainers, I want to see who would be a good maintainer"
"I want to promote/demote maintainers from a list of contributors."

Non-admins should not be able to promote/demote.