Fix donwload subs for non youtube videos and non-en language.

Check for status in test.
Add test for track url.
Fix test to use cookie.
Use download_track in feature file.
Add new tests for get_transcript.
Add html5 test.
Add content check.
Add tests for available_translations dispatch.
Disable flaky cms test.
Add  w/o english transcript tests.
Add to changelog.
Merge pull request #2786 from edx/alex/fix_download_html5_subs

Fix donwload subs for non youtube videos and non-en language.