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.
Fix acceptance tests.
Add query parameter to translation dispatch.
Response to format parameter of translatin GET request.
Fix style.
Fix typo.
Fix syntax errors.