Additional tags for users other than "admin"

The frontend issue for this is in pleroma-fe#94 (closed)

This would give us additional metadata like 'admin', 'moderator', etc.

Should probably be a new array type field and not just in 'info' because we probably want to filter on it.

This will give us the ability to filter groups of users on a server as well as easily identify more information about a user on the local instance. This information is not relevant to any part of the federation.