Better way of testing exceptions
More rigorous testing of duped pages ordering

[#52734615] showed that the previous tests of page ordering in duplicated activities were a bit naive. This is better.

https://www.pivotaltracker.com/story/show/52734615
Explicitly sets position of page in duped activity

To fix [#52734615]. The << method was unreliable, so we explicitly set the position value after adding the duped page to the new activity.

https://www.pivotaltracker.com/story/show/52734615
rm commented vestigial code
Using rack-environmental to add a warning banner
Minor controller spec cleanup
Commenting session key lookup
Ensure pages without response keys in the URL show them

[#53582601] The code to redirect to an URL with the response key was already in the controllers, but this (apparently superfluous) line in application_controller.rb was fooling them into thinking they had URLs with response keys.

https://www.pivotaltracker.com/story/show/53582601