Update onboarding CTA impression logging.

* Simplify analysis of the onboarding CTA impressions by logging pageNS
  (wgNamespaceNumber) and action (wgAction)
* Log if the user wasn't prompted to find other pages that need editing
* Replace underscores with spaces in the values of the ctaType parameter

Change-Id: I7bf5bdc84c0cb6eb335e9058f05cc5c0e4feae81