title: Django admin field choices render as multiple checkbox
body: <p>I'm trying to customise my Django admin page. I want a single model field to render all the choices passed to it as a CheckboxSelectMultiple, but can't get it to work.</p>
  
  <p>Here is the relevant part of my <code>models.py</code>:</p>
  
  <pre><code>from django.db import models
  from django.contrib.auth.models import User
  from django.contrib import admin
  
  class Event(models.Model):
      GROUP_OWNER_CHOICES = (
              ('vcoe', "VCOE"),
              ('cssi', 'CSSI'),
              ('essc', "ESSC"),
              ('tmscc', "TMSCC"),
              ('inmc', "INMC"),
              ('ccs7', "CCS7"),
              ('ias', "IAS"),
          )
  
      group_owner = models.BooleanField(choices=GROUP_OWNER_CHOICES, blank=True, default=False)
  </code></pre>
  
  <p>I can't use the Django <code>formfield_overrides</code>(<a href="https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.form" rel="nofollow">https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.form</a>) because I have more than 1 BooleanField and would like to keep the other BooleanField's intact.</p>
  
  <p>Here is what I found from other questions</p>
  
  <pre><code>from django.contrib import admin
  from .models import Event
  from django import forms
  
  class EventGroupOwnerAdminForm(forms.ModelForm):
      def __init__(self, *args, **kwargs):
          super(EventGroupOwnerAdminForm, self).__init__(*args,**kwargs)
          self.fields["group_owner"].widget = forms.widgets.CheckboxSelectMultiple()
  
  class EventAdmin(admin.ModelAdmin):
      form = EventGroupOwnerAdminForm
  
  admin.site.register(Event , EventAdmin)
  </code></pre>
  
  <p>But the field simply doesnt render anything. Any help?</p>