Valera/add option transcripts remove onhover

Fix for bug <b>BLD-355: Transcript hovers over videoplayer itself for PKU videos</b> ( https://edx-wiki.atlassian.net/browse/BLD-355 ).

<b>Bug description:</b>

In PekingX video content in courseware (.e.g. here, in the Folklore course: https://courses.edx.org/courses/PekingX/02030330X/3T2013/courseware/84f53f6e12c44345896c14af69e1cbb6/adc82bfe78c44e5a92f4a33b576b4f43/1), the transcript covers the right side of the video screen, covering the video itself. This fades if you stop hovering over the player in the window and reappears when you do. If you click "turn on captions", the transcript appears on the right side, off the video, as it does for our courses up to now when first clicking on courseware (for example, here: https://courses.edx.org/courses/MITx/4.605x/2013_SOND/courseware/3b97e1ff73914f289ba3d1f55343754a/8084d006f7b54d79a5144c27cd672fae/).

The courses are going live on 9/23.

<b>Solution:</b>

Add a flag ("autohide-html5") that will enable/disable the functionality of auto-show/auto-hide of captions based on mouse movement. When the flag will be set to "False", then only when the mouse moves over the "CC" button, will the captions auto-show. They will auto-hide if the user doesn't continue to move over the "CC" button, or over the captions themselves.

For now the flag "autohide-html5" is hard-coded to "<b>False</b>". There is no option in the "Advanced Edit" editor to set this flag.

<b>Reviewers:</b>
- @polesye 
- @jmclaus