Split verification dashboard analytics

This completes [ECOM-725](https://openedx.atlassian.net/browse/ECOM-725). Events are added to the dashboard to track interaction with upgrade copy, clicks on upgrade buttons, and clicks on "verify now" buttons. In addition to some refinement to the way events which track clicks on links are fired, the categories assigned to virtual pageviews generated during the payment and verification flows have been updated after a conversation with marketing.

@AlasdairSwan and @wedaly, could you review this when you're able?