Add source chooser to media player

The source chooser exposes available video sources of different sizes
and qualities. This allows users to select the quality that best matches
their currently available bandwidth.

Test plan:

 - With the "Prefer HTML5 for videos" feature option ON
   - Visit a page with a video on it
   - Click the starburst icon to see the list of available sources
   - Choose sources and observe that the quality of the video changes
   - Changing sources should preserve the play/pause state, timeline
     position, and playback speed
   - Multiple players on a page should not affect each other
   - You should not see this control show up for audio players
 - With the "Prefer HTML5 for videos" feature OFF:
   - Same as above, except that changing sources should only preserve the
     play/pause state (timeline and playback speed not available in the flash
     player)

Change-Id: I1c99a8e784904eed687f827164eed7e9d7e8c26c
Reviewed-on: https://gerrit.instructure.com/38639
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Paul Hinze <paulh@instructure.com>