Also allow system_user to send pm's even if enable_private_messages is disabled

lib/guardian.rb

@@ -210,7 +210,9 @@ class Guardian
     # Have to be a basic level at least
     @user.has_trust_level?(:basic) &&
     # PMs are enabled
-    (SiteSetting.enable_private_messages || @user.username == SiteSetting.site_contact_username)
+    (SiteSetting.enable_private_messages ||
+      @user.username == SiteSetting.site_contact_username ||
+      @user == Discourse.system_user)
   end
 
   private

spec/components/guardian_spec.rb

@@ -136,9 +136,10 @@ describe Guardian do
         Guardian.new(user).can_send_private_message?(another_user).should be_false
       end
 
-      it "returns true for the contact user" do
+      it "returns true for the contact user and system user" do
         SiteSetting.stubs(:site_contact_username).returns(user.username)
         Guardian.new(user).can_send_private_message?(another_user).should be_true
+        Guardian.new(Discourse.system_user).can_send_private_message?(another_user).should be_true
       end
     end
   end