Refactor video volume control.
i18n: More extractable & localizable strings in xmodules
Merge pull request #3678 from edx/anton/refactor-volume-control

Video: refactor volume control.
Merge pull request #3878 from louyihua/master

i18n: More extractable & localizable strings in xmodules
i18n: Fix an i18n string extraction error in 02_html5_video.js

Replace a string concation to a single string to prevent the partial extraction of the string.