Add caching for ApplicationSetting, Ci::ApplicationSetting.

ApplicationSetting.current was called in every pages, cache it and expires it after it updated.

This changes will avoid a SQL query in every pages (~0.3 - 0.5ms).

```SQL
SELECT  "application_settings".* FROM "application_settings"   ORDER BY "application_settings"."id" DESC LIMIT 1
```

1 files changed, 7 insertions, 1 deletions

diff --git a/app/models/application_setting.rb b/app/models/application_setting.rb
index 266045f7afa..5a9e55a95f4 100644
--- a/app/models/application_setting.rb
+++ b/app/models/application_setting.rb
@@ -68,8 +68,14 @@ class ApplicationSetting < ActiveRecord::Base
     end
   end
 
+  after_commit do
+    Rails.cache.write('application_setting.last', self)
+  end
+
   def self.current
-    ApplicationSetting.last
+    Rails.cache.fetch('application_setting.last') do
+      ApplicationSetting.last
+    end
   end
 
   def self.create_from_defaults