Add playback speed selection to the HTML5 media player

Test plan:

 - Ensure the "Prefer HTML5 for video playback" feature is enabled
 - Navigate to a page with a Canvas video player
 - Observe the additional speed control present to the right of the
   subtitle selector
 - Selecting speeds should change the playback speed of the video
 - Test various combinations of interaction, changing the speed while
   the video is playing, paused, after it has completed - everything
   should behave according to reasonable expectations
 - Repeat the test for an audio player
 - Note that Firefox will often fall back to a Flash-based player, which
   does not support playback speed control. This Firefox behavior is a
   separate issue which will be addressed in other commits.
 - Ensure that multiple players on a page work properly together

Change-Id: I322368140fa362bcaf47b2e4d8236c25e148a91a
Reviewed-on: https://gerrit.instructure.com/38479
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Ben Hutchings <benh@instructure.com>