default html5 first videos feature flag to ON

this was flipped to "on" for every root account with the notorious
rollout, so it should be the default going forward

Change-Id: I7c56e18ebe24560a1d23944a2530844415e01c00
Reviewed-on: https://gerrit.instructure.com/43977
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>