Fix select2 image assets

This commit takes the select2.css file from node_modules/select2
and alters it slightly to allow sprockets to recognize image asset
paths and include them in our production assets.

app/assets/stylesheets/application.scss

@@ -13,7 +13,7 @@
 // directory.
 @import '@gitlab/at.js/dist/css/jquery.atwho';
 @import 'dropzone/dist/basic';
-@import 'select2/select2';
+@import 'select2';
 
 // GitLab UI framework
 @import 'framework';