A feature flag to toggle html5/flash first video playback

this is a simple feature flag that will let us
toggle between:
a. trying html5 first then falling back to flash
   if not enabled or trying flash first.
b. trying flash first, then falling back to html5
   if unavailable.

test plan:
1. with feature flag not turned on, watch a video.
it should use flash to play it.
2. turn on the feature flag for domain_root_account
3. watch a video, it should use html5

Change-Id: I08aab043f89d049863dcbbd49aa1c4fcd8dc2895
Reviewed-on: https://gerrit.instructure.com/34248
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: David Julia <djulia@pivotallabs.com>
Reviewed-by: Paul Hinze <paulh@instructure.com>
Product-Review: Ben Hutchings <benh@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>