Found a bug? Please fill out the sections below. 👍

### Issue Summary

A summary of the issue.

### Steps to Reproduce

It's essential that you provide enough information for someone else to replicate the problem you're seeing. Simply describing something that's broken on your current project is not enough!

1. (for example) Start a new project with `wagtail start myproject`
2. Edit models.py as follows...
3. ...

Any other relevant information. For example, why do you consider this a bug and what did you expect to happen instead?

### Technical details

* Python version: Run `python --version`.
* Django version: Look in your requirements.txt, or run `pip show django | grep Version`.
* Wagtail version: Hover over the Wagtail bird in the admin, or run `pip show wagtail | grep Version:`.
* Browser version: You can use http://www.whatsmybrowser.org/ to find this out.
