* Add error handler on save video to avoid creating sjson
* Support transcripts without edx_video_id in definition_to_xml
* When copying a video from a library to a course: Create a new edx_video_id
* Save transcripts as static assets in a video in a library when adding a new transcript.
* Delete transcripts as static assets in a video in a library when deleting transcripts.
* Support download transcript in a video in a library.
* Support replace transcript in a video in a library.
* Support updating transcripts in video in a library.
* Refactor the code of downloading YouTube transcripts to enable this feature in libraries.
* Support copy from a library to a course and a course to a library.
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