Check for a custom css file to help customization of instances (#1368)

* User can create a custom.scss to customize their instance without modifying gitted files.

* Add documentation for customization.

* Forgot the helper file

* Fix Style to pass codeclimate

* Requests from maintainer.

app/views/layouts/application.html.haml

@@ -17,7 +17,7 @@
         = ' - '
       = site_title
 
-    = stylesheet_link_tag 'application', media: 'all'
+    = stylesheet_link_tag stylesheet_for_layout, media: 'all'
     = csrf_meta_tags
 
     = yield :header_tags