FIX: Better handling for toggling must_approve_users

If you turn it on now, default all users to approved since they were
previously. Also support approving a user that doesn't have a reviewable
record (it will be created first.)

This also includes a refactor to move class method calls to
`DiscourseEvent` into an initializer. Otherwise the load order of
classes makes a difference in the test environment and some settings
might be triggered and others not, randomly.

app/serializers/admin_user_serializer.rb

@@ -14,8 +14,7 @@ class AdminUserSerializer < AdminUserListSerializer
   has_one :single_sign_on_record, serializer: SingleSignOnRecordSerializer, embed: :objects
 
   def can_approve
-    reviewable = ReviewableUser.find_by(target: object)
-    reviewable.present? && reviewable.actions_for(scope).has?(:approve)
+    scope.can_approve?(object)
   end
 
   def include_can_approve?