Working on shaarli/Shaarli#224

Working on shaarli/Shaarli#224

I reviewed character escaping everywhere with the following ideas:

  * use a single common function to escape user data: `escape` using `htmlspecialchars`.
  * sanitize fields in `index.php` after reading them from datastore and before sending them to templates.
  	It means no escaping function in Twig templates.
    2 reasons:
    * it reduces risks of security issue for future user made templates
    * more readable templates
  * sanitize user configuration fields after loading them.