title: Multiselect form in Django Admin
body: <p>I'm attempting to add a multi-select widget to my admin view that corresponds to a char-field.</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class Campaign(models.Model):
      name = models.CharField(max_length=128)
      start_date = models.DateTimeField(default=timezone.now)
      end_date = models.DateTimeField(blank=True, null=True)
      budget = models.IntegerField()
      target = models.IntegerField()
      component_choices = models.CharField(max_length=128, null=True, blank=True)
  </code></pre>
  
  <p><strong>admin.py</strong></p>
  
  <pre><code>class CampaignAdmin(admin.ModelAdmin):
      fieldsets = (
          ('Main', {
              'fields': ('name', ('start_date', 'end_date',),),
          }),
          ('Budget', {
              'fields': ('budget', 'target',),
          }),
          ('Payment', {
              'fields': ('payment_terms', 'payment_initial', 'payment_per_mvm'),
          }),
      )
  
      form = ComponentChoicesForm
  
  admin.site.register(Campaign, CampaignAdmin)
  </code></pre>
  
  <p><strong>forms.py</strong></p>
  
  <pre><code>class ComponentChoicesForm(ModelForm):
  
      component_type_choices = tuple(
          (x.id, str(x))
          for x in ComponentType.objects.all()
      )
  
      class Meta:
          model = ComponentType
          fields = '__all__'
  
      available_components = forms.MultipleChoiceField(
          choices=component_type_choices)
  </code></pre>
  
  <p>However the above code only displays the <code>fieldsets</code> and not the <code>ComponentTypeChoices</code> form at all. My intent is to have the user select from the choices i generate in <code>forms.py</code> in the <code>admin.py</code> in order to populate the <code>Campaign.component_choices</code> field.</p>