Add new welcome message step

lib/introduction_updater.rb

@@ -0,0 +1,42 @@
+class IntroductionUpdater
+
+  def initialize(user)
+    @user = user
+  end
+
+  def get_summary
+    summary_from_post(find_welcome_post)
+  end
+
+  def update_summary(new_value)
+    post = find_welcome_post
+    return unless post
+
+    previous_value = summary_from_post(post).strip
+
+    if previous_value != new_value
+      revisor = PostRevisor.new(post)
+
+      remaining = post.raw.split("\n")[1..-1]
+      revisor.revise!(@user, raw: "#{new_value}\n#{remaining.join("\n")}")
+    end
+
+  end
+
+protected
+
+  def summary_from_post(post)
+    return post ? post.raw.split("\n").first : nil
+  end
+
+  def find_welcome_post
+    welcome_topic = Topic.where(slug: 'welcome-to-discourse').first
+    return nil unless welcome_topic.present?
+
+    post = welcome_topic.posts.where(post_number: 1).first
+    return nil unless post.present?
+
+    post
+  end
+
+end