Redirect guests to signon

Given I am a guest that is not logged in
When I attempt to preview content
Then I would like to be redirected to the signon page
And I can login
And I will be redirected back to the preview content

Based on a comment discussion with @bradleywright 
https://github.com/alphagov/whitehall/commit/52281e7a0011214319f1e70bb33a24d9b4325da0#commitcomment-3022018

I don't know the application, so I don't know if the change
is valid, but seems to work for me.