title: Display additional content panel based on boolean block in Wagtail Editor Interface
body: <p>I want to add a boolean block to my Wagtail model and show a content panel field only if it is checked. I have figured out how to add a boolean block and render content in the template based on its value, but not how to do control the editor interface with it. Here is my model. I want to show the heldover_from, date chooser block, only when heldover boolean is checked.</p>
  
  <pre><code>class AgendaPage(Page):
  author= models.CharField(max_length=255)
  date = models.DateField('Post date')
  agenda = StreamField([
      ('agenda_item', blocks.StreamBlock([
          ('item_title', blocks.TextBlock()),
          ('item_text', blocks.TextBlock()),
          ('mtg_doc', blocks.StructBlock([
              ('doc_description', blocks.TextBlock()),
              ('doc_link', blocks.TextBlock()),
              ('submitted_late', blocks.BooleanBlock(required=False, help_text='Submitted Late')),
              ('heldover', blocks.BooleanBlock(required=False, help_text='Held Over')),
              ('heldover_from', blocks.DateBlock(required=False, help_text="Held Over From")
  
          ]))
        ]
      ))
  ])
  
  
  
  
  content_panels = Page.content_panels + [
      FieldPanel('author'),
      FieldPanel('date'),
      StreamFieldPanel('agenda'),
  
  ]
  </code></pre>
  
  <p>(And after I figure this out, I want to know if I can make it required, but only if heldover is checked, not for the whole streamblock)</p>
  
  <pre><code>{% for block in self.agenda %}
    {% if block.block_type == "agenda_item" %} {# will always be true, but included here for clarity #}
    &lt;li&gt;
      {% for subblock in block.value %}
          {% if subblock.block_type == "item_title" %}
              &lt;h2&gt;{{subblock.value}}&lt;/h2&gt;
           {% elif subblock.block_type == "item_text" %}
                   &lt;p&gt;{{subblock.value}}&lt;/p&gt;
           {% elif subblock.block_type == "mtg_doc" %}
                  &lt;p&gt;&lt;a href="{{subblock.value.doc_link}}"&gt;{{subblock.value.doc_description}}&lt;/a&gt;&lt;br /&gt;
              {% ifequal subblock.value.submitted_late True %}
               (Submitted Late)
              {% endifequal %}
                  &lt;/p&gt;
          {% endif %}
      {% endfor %}
  &lt;/li&gt;
    {% endif %}
  {% endfor %}
  </code></pre>