Sending a GA event when an activity is completed

[#39645869] This sends an Activity Completed event to Google Analytics when the user reaches a "terminal step", which is defined as a step on with no defaultBranch, no responseBranches, and either no submissibilityCritereon or isFinalStep set to true. (And also that the activityPageController returns true for 'isLastPage'.)
Merge branch 'activities-4-5-dev' into activities-4-5