Make the "invalid authenticity token" 403 error pretty like the others

- Users *shouldn't* see this, so don't include any extra information.

![screen shot 2014-12-18 at 12 16 34](https://cloud.githubusercontent.com/assets/355033/5487962/c3702a58-86af-11e4-8a07-c7c2f1440051.png)

I couldn't make either this or https://github.com/alphagov/transition/pull/480/files#diff-e203932987375b93e1b7d8c77e0fa2d3L550 pass (presumably because there's no body text, just a heading), hence their removal.