Remind author do something only when he has permission

Refer: gitlab-cn/gitlab#1206 (comment 603406)

1. The operation "Add label" need have group permission, so we need filter reminders when the author is not in the group gitlab-cn members.
2. Search other processors, deal with the reminders conflicts with permissions, already known: TypeLabelReminder, ChildLabelReminder

/cc @qianzhangxa