Allow a configurable delay to keep diary entries from being published in feeds

This is to provide another tool to help spam fighting.

The configuration value is set to zero hours for new deploys in
order to be 'least surprising'. You can change the setting without
interfering with your test suite.

test/controllers/diary_entries_controller_test.rb

@@ -664,6 +664,18 @@ class DiaryEntriesControllerTest < ActionController::TestCase
     assert_match "<title>&lt;script&gt;</title>", response.body
   end
 
+  def test_feed_delay
+    create(:diary_entry, :created_at => 7.hours.ago)
+    create(:diary_entry, :created_at => 5.hours.ago)
+    get :rss, :params => { :format => :rss }
+    assert_select "rss>channel>item", :count => 2
+
+    with_diary_feed_delay(6) do
+      get :rss, :params => { :format => :rss }
+      assert_select "rss>channel>item", :count => 1
+    end
+  end
+
   def test_show
     user = create(:user)
     suspended_user = create(:user, :suspended)
@@ -897,4 +909,13 @@ class DiaryEntriesControllerTest < ActionController::TestCase
       assert_select "a[href=?]", "/user/#{ERB::Util.u(entry.user.display_name)}/diary/#{entry.id}"
     end
   end
+
+  def with_diary_feed_delay(value)
+    diary_feed_delay = Settings.diary_feed_delay
+    Settings.diary_feed_delay = value
+
+    yield
+
+    Settings.diary_feed_delay = diary_feed_delay
+  end
 end