Video: allow multiple transcripts

Ticket: [BLD-642](https://edx-wiki.atlassian.net/browse/BLD-642)
XML should look like:
```xml
<video youtube="1.00:gnqth8uWzO8" display_name="Biology Demonstration" >
     <transcript language="de" src="my_german_translation.srt" />
     <transcript language="zh" src="my_mandarin_translation.srt" />
     <transcript language="ru" src="my_russian_translation.srt" />
</video>
```
When the student hovers over the 'cc' button then a menu appears over the 'cc' button with languages.
The show/hide transcript functionality remains present upon clicking the 'cc' button
The "download transcript" button should download whichever transcript is currently selected by the student.

@nedbat, @singingwolfboy, @valera-rozuvan, @frrrances  please review.

Current coverage is 84% and it is different from jenkins results because coverage.py configured such that tests in LMS/CMS don't count coverage for modules in common/lib.
common/lib/xmodule/xmodule/video_module/video_module.py (96.0%)
common/lib/xmodule/xmodule/video_module/transcripts_utils.py (49.7%)
common/lib/xmodule/xmodule/video_module/video_utils.py (100%)
Transcript_utils is covered by acceptance tests.