title: Many to many fields in widget form
body: <p>I have problem with display many to many field in form widget.
  Category is not display in template.
  Title is ok (is display) but category isn't - category is empty.
  What can I do to display many to many fields in my template form with multiplechoice checkboxes?
  Why I cant display article categories in widget form?</p>
  <p><strong>MODELS.py</strong></p>
  <p>article model:</p>
  <pre><code>class Article(Created, HitCountMixin):
      title = models.CharField(max_length=120)
      author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
      category = models.ManyToManyField(ArticleCategory, related_name='articles')
  </code></pre>
  <p>category model:</p>
  <pre><code>class ArticleCategory(Created):
      category_name = models.CharField(max_length=128)
      slug = models.SlugField(null=False, unique=False)
  </code></pre>
  <p><strong>VIEWS:</strong></p>
  <pre><code>class UpdateArticleView(LoginRequiredMixin, UpdateView):
      template_name = 'news/update_article.html'
      form_class = EditArticleForm
      model = Article
  
      def get_success_url(self):
          pk = self.kwargs[&quot;pk&quot;]
          slug = self.kwargs['slug']
          return reverse_lazy(&quot;news:article_detail&quot;, kwargs={'pk': pk, 'slug': slug})
  </code></pre>
  <p><strong>FORMS.py</strong></p>
  <pre><code>class AddArticleForm(forms.ModelForm):
  
  title = forms.CharField(
      label=&quot;Tytuł&quot;,
      max_length=120,
      help_text=&quot;Tytuł newsa&quot;,
      widget=forms.TextInput(attrs={&quot;class&quot;: &quot;form-control form-control-lg pr-5 shadow p-1 mb-1 bg-white rounded&quot;}),
      required=True,
  )
  
  
  category = forms.MultipleChoiceField(
      widget=forms.CheckboxSelectMultiple,
  )
  </code></pre>
  <p>And in my <strong>HTML TEMPLATE:</strong></p>
  <pre><code>    &lt;form method=&quot;post&quot; enctype='multipart/form-data'&gt;
          {% csrf_token %}
          {{ form.media }}
          {#        {%  crispy form %}#}
          {{ form|crispy }}
          &lt;button type=&quot;submit&quot; class=&quot;btn btn-outline-primary&quot;&gt;EDYTUJ NEWS&lt;/button&gt;
      &lt;/form&gt;
  </code></pre>