FIX: Wizard tries harder to find existing Welcome Topic

The wizard searches for:

* a topic that with the "is_welcome_topic" custom field
* a topic with the correct slug for the current default locale
* a topic with the correct slug for the English locale
* the oldest globally pinned topic

It gives up if it didn't find any of the above.

spec/lib/introduction_updater_spec.rb

@@ -0,0 +1,56 @@
+require 'rails_helper'
+require 'introduction_updater'
+
+describe IntroductionUpdater do
+  describe "#get_summary" do
+    subject { IntroductionUpdater.new(Fabricate(:admin)) }
+
+    let(:welcome_post_raw) { "lorem ipsum" }
+    let(:welcome_topic) do
+      topic = Fabricate(:topic)
+      Fabricate(:post, topic: topic, raw: welcome_post_raw, post_number: 1)
+      topic
+    end
+
+    it "finds the welcome topic by custom field" do
+      TopicCustomField.create(topic_id: welcome_topic.id, name: "is_welcome_topic", value: "true")
+      expect(subject.get_summary).to eq(welcome_post_raw)
+    end
+
+    context "without custom field" do
+      it "finds the welcome topic by slug using the default locale" do
+        I18n.locale = :de
+        welcome_topic.title = I18n.t("discourse_welcome_topic.title")
+        welcome_topic.save!
+
+        expect(subject.get_summary).to eq(welcome_post_raw)
+      end
+
+      it "finds the welcome topic by slug using the English locale" do
+        welcome_topic.title = I18n.t("discourse_welcome_topic.title", locale: :en)
+        welcome_topic.save!
+        I18n.locale = :de
+
+        expect(subject.get_summary).to eq(welcome_post_raw)
+      end
+
+      it "doesn't find the topic when slug_generation_method is set to 'none'" do
+        SiteSetting.slug_generation_method = :none
+        welcome_topic.title = I18n.t("discourse_welcome_topic.title")
+        welcome_topic.save!
+
+        expect(subject.get_summary).to be_blank
+      end
+
+      it "finds the oldest globally pinned topic" do
+        welcome_topic.update_columns(pinned_at: Time.zone.now, pinned_globally: true)
+
+        expect(subject.get_summary).to eq(welcome_post_raw)
+      end
+
+      it "doesn't find the topic when there is no globally pinned topic or a topic with the correct slug" do
+        expect(subject.get_summary).to be_blank
+      end
+    end
+  end
+end