Display transcript translations for student in their source language.
Fix acceptance tests.
Wait for big subtitles to render.
Set big timeout for big subtitles.
Fix flaky video tests: speed up youtube api loading and wait for controls.
Merge pull request #3136 from edx/alex/display_native_language_2

Alex/display native language 2
Merge pull request #3169 from edx/alex/fix_more_flaky_tests

Alex/fix more flaky tests
Refactor video caption module.