Tabbing from password field goes to Submit

When I log in to a site, my muscle memory typically goes:

- type username
- [TAB]
- type password
- [TAB]
- [Enter]

On the Wagtail log in page at the moment, the second [TAB] followed by [Enter] instead opens the 'Forgot password' page. I'm now bored of reading the Wagtail 'forgot password' page.

This pull request redresses that.