Specifically, a "I visited this page" event, with information on
which variant was seen. And "I clicked this button" events, for
our various calls to action.
- Adds a non-passing cert learner course exit screen
- Moves all the logic about what course-exit mode we're in into
a utility method in the course-exit folder
- Moves all the logic about how the 'Next' button should read into
a utility method in the course-exit folder