Update captions logic on front-end.

Ticket: [BLD-950](https://edx-wiki.atlassian.net/browse/BLD-950)

For testing use default video and following streams 
0.75:JMD_ifUUfsU
1.00:OEoXaMPEzfM
1.25:AKqURZnYqpk
1.50:DYpADpL7jAY


@auraz , @valera-rozuvan please review.