Story 1072: KeepGoing CTA

* Introduce sticky CTA which allows user to continue editing
* Show CTA after a successful first edit or when
campaign=mobile-keepgoing is present in query string
* Allow new editor to try other articles
* dismissing cta exits the campaign (although if you refresh
it will show again)
* EventLogging via mobileWebCta.js

Change-Id: I4c022df9634e55a6dedcf9237ba3318a66f5677b
Merge "Story 1072: KeepGoing CTA"