Fix #3846: Subscribed forum posts link to invalid URL in email updates.

Test: http://localhost/rails/mailers/user_mailer/forum_notice

app/views/user_mailer/forum_notice.html.erb

@@ -4,7 +4,7 @@
   <p>
     <strong><%= forum_post.creator_name %> said:</strong>
   </p>
-  <%= format_text(forum_post.body) %>
+  <%= format_text(forum_post.body, base_url: root_url) %>
   <br>
   <hr>
   <br>

test/mailers/previews/user_mailer_preview.rb

@@ -3,4 +3,12 @@ class UserMailerPreview < ActionMailer::Preview
     dmail = User.admins.first.dmails.first
     UserMailer.dmail_notice(dmail)
   end
+
+  def forum_notice
+    topic = ForumTopic.first
+    posts = topic.posts
+    user = topic.creator
+
+    UserMailer.forum_notice(user, topic, posts)
+  end
 end