Allow downloading transcriptions on watchLectureVideos page

[chirantan31]

No description provided.

[angrave]

Current implementation seems to get the transcriptions from the client
transcriptions = JSON.parse(request.body.transcriptions);
Better - get the transcriptions from the server and immediately stream it as content to be downloaded. There is no need to create a new file and then a secondary request to read that file.
https://stackoverflow.com/questions/10046039/nodejs-send-file-in-response
if( can-create-vtt-stream)
response.writeHead(200, {
"Content-Type": "application/octet-stream",
"Content-Disposition": "attachment; filename=" + captions_name_vtt
});
outputsstream.pipe(response);
} else {
response.writeHead(400, {"Content-Type": "text/plain"});
response.end("Requested Transcriptions do not exist or you do not have access);
}