This adds two new events "display" and "hover" which are not
recorded back to the server. The benefits of having these events
is that they are important events in the lifecycle of a hovercard.
This allows us to debug trackSubscribe and ensure we see the behaviour
we expect to see and in a future patchset will allow us to use these
events to drive the calculation of interaction time in one single location
(Sneak preview:
https://gerrit.wikimedia.org/r/316481 to get a feel for the why.)
Change-Id: I58eefc29444179fd245cfd722093dedea19455e8
