Add viewport meta

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/layouts/_head.html.haml

@@ -8,6 +8,8 @@
   = javascript_include_tag "application"
   = csrf_meta_tags
   = include_gon
+  :erb
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
 
   = render 'layouts/google_analytics' if extra_config.has_key?('google_analytics_id')