Because nobody else should need to do anything as cursed as what
`ScheduleScreen` was doing (which is 95% of the reason why the selection
box even broke there).
Seems a lot more reliable, and allows falling back to the "main" video
in cases which didn't support this previously.
A next step may be to allow every screen to support a video based on its
screen name, rather than specifying the local `TourneyVideo` every time.
Using an abstract property was awkward for this as it is being consumed
in the underlying constructor but could not be dynamically set in time from a
derived class.