FEATURE: support group owner, capable of controlling group membership

Group owners are regular users that can add or remove users to a group
The Admin UX allows admins to appoint group owners
The public group UX will display group owners first and unlock UI to
add and remove members

Group owners can only be appointed on non automatic groups
Group owners may not appoint another group owner

app/models/group.rb

@@ -7,9 +7,6 @@ class Group < ActiveRecord::Base
   has_many :categories, through: :category_groups
   has_many :users, through: :group_users
 
-  has_many :group_managers, dependent: :destroy
-  has_many :managers, through: :group_managers
-
   after_save :destroy_deletions
   after_save :automatic_group_membership
   after_save :update_primary_group
@@ -283,8 +280,8 @@ class Group < ActiveRecord::Base
     user.update_columns(primary_group_id: nil) if user.primary_group_id == self.id
   end
 
-  def appoint_manager(user)
-    managers << user
+  def add_owner(user)
+    self.group_users.create(user_id: user.id, owner: true)
   end
 
   protected