title: Django Admin ManyToManyField
body: <p>I've made a model (models.py):</p>
  
  <pre><code>class opetest(models.Model):
      name = models.CharField(max_length=200)
      author = models.ForeignKey(User, related_name='author')
      description = models.TextField(u'Test description', help_text = u'Some words about quiz')
      pub_date = models.DateTimeField('date published', blank=False)
      vacancies = models.ManyToManyField(Vacancy, blank=True)
      students = models.ManyToManyField(User, blank=True, related_name='opetests') #This field I want to edit on "User change page"
      estimate = models.IntegerField(default = 0, help_text = u'Estimate time in hours. \'0\' - unlimited')
  </code></pre>
  
  <p>then I try to add inline block to allow assign <code>opetest</code> on 'change user' page (admin.py):</p>
  
  <pre><code>class ProfileAdmin(UserAdmin):
      filter_horizontal = ('opetests',)
  
  admin.site.unregister(User)
  admin.site.register(User, ProfileAdmin)
  </code></pre>
  
  <p>And I got an error:</p>
  
  <pre><code>'ProfileAdmin.filter_horizontal' refers to field 'opetests' that is missing from model 'User'.
  </code></pre>
  
  <p>I want to show opetests like <em>Groups</em> on <em>change user</em> page. How can I achieve that?</p>