Upgrade Turbolinks to version 5.2.1

We didn't upgrade Turbolinks when we upgraded to Rails 5 so we didn't
upgrade too many things at the same time, and postponed it... until now
:).

Note upgrading Turbolinks fixes an issue with foundation's sticky when
using the browser's back and forward buttons. We're adding tests for
these scenarios.

Co-authored-by: Senén Rodero Rodríguez <senenrodero@gmail.com>

app/assets/javascripts/html_editor.js

@@ -17,6 +17,5 @@
     }
   };
 
-  $(document).on("page:before-unload", App.HTMLEditor.destroy);
-  $(document).on("page:restore", App.HTMLEditor.initialize);
+  $(document).on("turbolinks:before-cache", App.HTMLEditor.destroy);
 }).call(this);