Allows students to download the transcript of the video without timecodes

Ticket: [BLD-368](https://edx-wiki.atlassian.net/browse/BLD-368).

Change the track field to a dropdown that will allow students to download the transcript of the video without timecodes.

If `track` field exists show `track` field on front-end as not-editable and clearable. Dropdown `download_track` is a new field and it has value `True`. Student downloads transcript from `track` field as it is. If teacher want to change `track`, he should clear `track` field and upload transcript file. After that, students will available to download the transcript of the video without timecodes.

@nedbat , @auraz , @valera-rozuvan , @frrrances please review.