Delegation

Closes #38 (closed) and #39 (closed)

• Replaces primary key of invites with $not-the-secret-token to allow admin operations on invite without leaking the token
• Associates invite links with the creating user
• Adds ACL_SIGNUP_GROUP that allows non-admin users to create invite links for signup
• Introduces Role.moderator_group that, if set, makes its members role moderators
• Role moderators can create invite links that grant access to roles the creator is a moderator of
• Role moderators can edit the role description, view and remove role members