stageTimer in intro/exit stages

[JamesPHoughton]

We'd like to have something akin to the stageTimer for intro and exit stages, to show/hide elements, log durations, etc. We want this to be persistent across re-renders and page refreshes, but not persist across experiment sessions.

One way to do that would be that when a GenericIntroExitStep is loaded, save the current stage start time to localstorage (or the player object?) and reset that value when the step changes. (so maybe also set the current intro timer stage name)

[JamesPHoughton]

Setting player.set("localStageStartTime", Date.now()) in the GenericIntroExitStep, and forcing the same to unknown when in game steps. Make this only happen the first render of the stage by manually updating and tracking the progressLabel.