Use HTML5 for videos by default so subtitles work in fullscreen

also,make subtitle text bigger when in fullscreen

fixes: CNVS-12293

videos should play back like they used to but this commit
switches back to using html5 by default instead of flash
because that is the only way that subtitles will show up in
fullscreen mode

test plan:
in all the browsers we support (in particular: ie10, android
and mobile safari):
* try playing a video that you have uploaded a subtitle track for.
* the playback should work
* the subtitles should show up when in normal-sized mode
* press the fullscreen button, the video should go into
  fullscreen correctly and still show the subtitles
  (and the subtitles should be bigger in fullscreen mode)

Change-Id: I5b77f58133fb9081909771d50bb8f0a91db23d63
Reviewed-on: https://gerrit.instructure.com/34016
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Paul Hinze <paulh@instructure.com>
Product-Review: Ben Hutchings <benh@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>