title: Set Wagtail BooleanField value to True, disabled
body: <p>My team and I are building a "page builder" using the StreamField capabilities in <code>Wagtail 2.5.1</code>. We've built a number of modules which will be used by our marketing team. One of the module requests is a "divider bar".</p>
  
  <p>I've created it with some very simple code:</p>
  
  <pre><code>class DividerBlock(blocks.StructBlock):
  
      include_divider = blocks.BooleanBlock(
          required=False,
          help_text='Display a divider bar',
          default=True,
      )
  
      class Meta:
          label = 'Divider Module'
          icon = 'horizontalrule' 
  </code></pre>
  
  <p>I don't actually <em>need</em> the checkbox because I'm simply looking for the existence of this module on the page, and we'll output the HR. But if I don't include at least one control, the streamfield shows an empty space, so a checkbox makes sense. However I want to force the checkbox to default to true (already done) AND be disabled or readonly so that the user cannot change it.</p>
  
  <p>I've tried both disabled and readonly as properties in the BooleanBlock init but neither are working. Is there a way to accomplish this?</p>