While show_homepage_promo_video would previously toggle the control to
play the welcome video on and off, the YouTube embed iframe would be
included in index.html unconditionally. This created the ugly
situation that YouTube would try to pull in a non-existent video, and
that a DoubleClick tracking beacon would be introduced to the start
page for no reason.

Instead, make sure show_homepage_promo_video toggles the entire modal
video section on and off, suppressing it altogether unless
show_homepage_promo_video is set.