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.

lib/introduction_updater.rb

@@ -22,25 +22,47 @@ class IntroductionUpdater
     end
   end
 
-protected
+  protected
 
   def summary_from_post(post)
     return post ? post.raw.split("\n").first : nil
   end
 
   def find_welcome_post
-    topic_id = TopicCustomField.where(name: "is_welcome_topic").where(value: "true").pluck(:topic_id)
-    unless topic_id.present?
-      topic_id = Topic.listable_topics.where(slug: 'welcome-to-discourse').pluck(:id)
+    topic_id = TopicCustomField
+      .where(name: "is_welcome_topic", value: "true")
+      .pluck(:topic_id)
+
+    if topic_id.blank?
+      title = I18n.t("discourse_welcome_topic.title")
+      topic_id = find_topic_id(title)
+    end
+
+    if topic_id.blank?
+      title = I18n.t("discourse_welcome_topic.title", locale: :en)
+      topic_id = find_topic_id(title)
+    end
+
+    if topic_id.blank?
+      topic_id = Topic.listable_topics
+        .where(pinned_globally: true)
+        .order(:created_at)
+        .limit(1)
+        .pluck(:id)
     end
 
     welcome_topic = Topic.where(id: topic_id).first
-    return nil unless welcome_topic.present?
+    return nil if welcome_topic.blank?
 
-    post = welcome_topic.posts.where(post_number: 1).first
-    return nil unless post.present?
-
-    post
+    welcome_topic.first_post
   end
 
+  def find_topic_id(topic_title)
+    slug = Slug.for(topic_title, nil)
+    return nil if slug.blank?
+
+    Topic.listable_topics
+      .where(slug: slug)
+      .pluck(:id)
+  end
 end