FIX: Topic#invite_group failed to notify users on newly created topic (#12255)

When `PostCreator` creates a new topic it loads the `allowed_groups` of the topic. `Fabricate` doesn't do that and that's why the existing spec worked even though it should have failed, because `PostAlerter#notify_group_summary` didn't create a notification for a non-fabricated topic.

`Topic#invite_group` added a new `TopicAllowedGroup` record without reloading `Topic.allowed_groups`. A subsequent call to `PostAlerter#notify_group_summary` didn't work because it didn't find the invited group in the topic's `allowed_groups` association.

spec/models/topic_spec.rb

@@ -909,7 +909,15 @@ describe Topic do
   context 'private message' do
     let(:coding_horror) { Fabricate(:coding_horror) }
     fab!(:evil_trout) { Fabricate(:evil_trout) }
-    let(:topic) { Fabricate(:private_message_topic, recipient: coding_horror) }
+    let(:topic) do
+      PostCreator.new(
+        Fabricate(:user),
+        title: "This is a private message",
+        raw: "This is my message to you-ou-ou",
+        archetype: Archetype.private_message,
+        target_usernames: coding_horror.username
+      ).create!.topic
+    end
 
     it "should integrate correctly" do
       expect(Guardian.new(topic.user).can_see?(topic)).to eq(true)