title: How to put data from ManyToManyField to choices in MultipleChoiceField
body: <p>I have a form:</p>
  
  <pre><code>*# dialogues.forms*
  
  class CreateConferenceForm(forms.Form):
      ...
      participants = forms.MultipleChoiceField(choices=?)
      ...
  </code></pre>
  
  <p>And I need to put in choices argument instead of "?" data from friends field in this model:</p>
  
  <pre><code>*# persons.models*
  
  class Person(User):
      ...
      friends = models.ManyToManyField(
          'self',
          related_name='+',
      ) 
      ...
  </code></pre>
  
  <p>How can I do it?</p>