FEATURE: Send a private message when a group membership is accepted (PR #9822)

Instead of sending the usual notification, it will send a private message instead, which in turn may send an email.

GitHub

      title: "You've been accepted into @%{group_name}"
        Your request to enter @%{group_name} has been accepted and you are now a member.
      topic = Topic.last
      expect(topic.title).to eq(I18n.t('groups.request_accepted_pm.title', group_name: group.name))
      expect(topic.first_post.raw).to eq(I18n.t('groups.request_accepted_pm.body', group_name: group.name).strip)

This change doesnโ€™t seem to be required.

I would also test that topic is a private message.

LGTM :+1:

I would play it safe and ensure the topic that is created is indeed a private message.