Add multiple transcripts editor.

Fix donwload subs for non youtube videos and non-en language - continue.
Add acceptance tests.
Add detetion of assets on request.
Updated docstring.
Add fixes and acceptance tests.
Fix acceptance tests.
Update docsrtings and cleanup code, resful for language_id.
Specify exception type in POST.
Fix url in upload module.
Improve exception handling.
Remove 'en' and catching in editable_metadata.
Fix typo.
Add tests for translation DELETE dispatch.
Fix dispatches im tests.
Remove debugger.
Update acceptance tests.
Fix comments.
Add tests for POST dispatch.
Move descriptor.get_context test to lms tests.
Clean comments in PR.
Fix test_download_video_is_explicitly_set test.
Fix forgotten rename.
Fix unit tests after rename.