title: Tags input with symfony/form
body: <p>I need to implement a tags input like <a href="https://select2.github.io/examples.html#tags" rel="nofollow">https://select2.github.io/examples.html#tags</a>. I'm not quite sure how to implement the server-side part with symfony/form. I couldn't find any existing implementation and I'm not sure if I should try to somehow hack the ChoiceType or try to implement it separately.</p>
  
  <p>Can you help me? Did I miss any existing solution?</p>