title: Wagtail profile avatar into ImageRenditionField serializer
body: <p>I want to serialize the profile avatar from Wagtail admin with ImageRenditionField.
  Basing on the answer from the <a href="https://stackoverflow.com/questions/54952994/how-to-render-the-user-profile-avatar-image-into-the-wagtail-template">question</a> I tried this:</p>
  <pre><code># models.py
  class User(AbstractUser):
  
      def get_avatar(self):
          return self.wagtail_userprofile.avatar
  
  
  # serializers.py
  class UserSerializer(serializers.ModelSerializer):
  
      avatar = ImageRenditionField('width-190', source='get_avatar')
      class Meta:
          model = User
          fields = ['id', 'username', 'first_name', 'last_name', 'avatar']
  </code></pre>
  <p>I got this:</p>
  <pre><code>Django Version: 3.1.4
  Exception Type: AttributeError
  Exception Value:    
  'ImageFieldFile' object has no attribute 'get_rendition'
  </code></pre>
  <p>How do I throw user avatar into ImageRenditionField?</p>