Add default_external_links_in_new_tab site setting

app/models/user.rb

@@ -63,6 +63,7 @@ class User < ActiveRecord::Base
   before_save :ensure_password_is_hashed
   after_initialize :add_trust_level
   after_initialize :set_default_email_digest
+  after_initialize :set_default_external_links_in_new_tab
 
   after_save :update_tracked_topics
 
@@ -596,6 +597,12 @@ class User < ActiveRecord::Base
     end
   end
 
+  def set_default_external_links_in_new_tab
+    if has_attribute?(:external_links_in_new_tab) && self.external_links_in_new_tab.nil?
+      self.external_links_in_new_tab = !SiteSetting.default_external_links_in_new_tab.blank?
+    end
+  end
+
   private
 
   def previous_visit_at_update_required?(timestamp)

config/locales/server.en.yml

@@ -752,6 +752,7 @@ en:
     allow_uploaded_avatars: "Allow users to upload their custom avatars"
     allow_animated_avatars: "Allow users to use animated gif for avatars. WARNING: it is highly recommended to run the avatars:regenerate rake task after changing that setting."
     default_digest_email_frequency: "How often users receive digest emails by default. They can change this setting in their preferences."
+    default_external_links_in_new_tab: "Open external links in a new tab. Users can change this in their preferences."
 
     detect_custom_avatars: "Whether or not to check that users have uploaded custom avatars"
     max_daily_gravatar_crawls: "The maximum amount of times Discourse will check gravatar for custom avatars in a day"

config/site_settings.yml

@@ -126,6 +126,7 @@ users:
     client: true
     default: 14
   delete_all_posts_max: 15
+  default_external_links_in_new_tab: false
 
 posting:
   min_post_length:

db/migrate/20140102194802_remove_default_from_external_links_in_new_tab.rb

@@ -0,0 +1,9 @@
+class RemoveDefaultFromExternalLinksInNewTab < ActiveRecord::Migration
+  def up
+    change_column_default :users, :external_links_in_new_tab, nil
+  end
+
+  def down
+    change_column_default :users, :external_links_in_new_tab, false
+  end
+end