title: FieldDoesNotExist when adding content_panels fields to Page
body: <p>My head hurts from beating it against this wall ... Why can't the content panels recognize the fields in my DetailPage?  Everything works fine until I add the FieldPanels to the content_panels. </p>
  
  <p>Oh, this is using the 2.0 version of wagtail also.  This doesn't seem to be related to the field types, event a simple CharBlock triggers this error.</p>
  
  <p><strong>STACK TRACE</strong></p>
  
  <pre><code>Performing system checks...
  
  Unhandled exception in thread started by &lt;function check_errors.&lt;locals&gt;.wrapper at 0x0000005F7CDB5598&gt;
  Traceback (most recent call last):
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\models\options.py", line 566, in get_field
      return self.fields_map[field_name]
  KeyError: 'links'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\django\utils\autoreload.py", line 225, in wrapper
      fn(*args, **kwargs)
  ...
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 766, in get_edit_handler
      return edit_handler.bind_to_model(cls)
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 130, in bind_to_model
      new.on_model_bound()
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 275, in on_model_bound
      for child in self.children]
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 275, in &lt;listcomp&gt;
      for child in self.children]
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 130, in bind_to_model
      new.on_model_bound()
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 275, in on_model_bound
      for child in self.children]
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 275, in &lt;listcomp&gt;
      for child in self.children]
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 130, in bind_to_model
      new.on_model_bound()
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\wagtail\admin\edit_handlers.py", line 479, in on_model_bound
      self.db_field = self.model._meta.get_field(self.field_name)
    File "C:\Users\RichardK\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\models\options.py", line 568, in get_field
      raise FieldDoesNotExist("%s has no field named '%s'" % (self.object_name, field_name))
  django.core.exceptions.FieldDoesNotExist: DetailPage has no field named 'links'
  </code></pre>
  
  <p><strong>models.py</strong>
      from django.db import models</p>
  
  <pre><code>from wagtail.core.models import Page
  from wagtail.core.fields import RichTextField
  from wagtail.core import blocks
  from wagtail.core.blocks import ListBlock, URLBlock, StructBlock, TextBlock, CharBlock
  from wagtail.admin.edit_handlers import FieldPanel
  from wagtail.search import index
  
  
  class DetailPage(Page):
      """
      Two column detail page with quick links on the left
      """
      links = blocks.ListBlock(blocks.URLBlock(), null=True, blank=True)
      body = RichTextField(blank=True)
  
      content_panels = Page.content_panels + [
          FieldPanel('links', classname="full"),
          FieldPanel('body')
      ]
  </code></pre>