PHRAS-3934 fix videotools subtitle timeline #431

Job	Run time
build phraseanet-nginx / build	4m 30s
build phraseanet-fpm / build	5m 23s
build phraseanet-elasticsearch / build	34s
build phraseanet-db / build	36s
build phraseanet-setup / build	4m 47s
build phraseanet-worker / build	4m 36s
unit test	13m 6s
	33m 32s

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