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

### Issue Summary

A summary of the issue.


### Steps to Reproduce

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?

* I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: (yes / no)


### 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.