Video: Fix download transcript behavior. 

DT - Download Transcript;
DA - Transcript Download Allowed.

The following behavior is provided:
1) Both fields are editable and always present;
2) If DA is False, "Download Transcript" button is disabled;
3) If DA is True and DT is not empty, "Download Transcript" button is enabled and link from DT field is used;
4) If DA is True and DT is empty, "Download Transcript" button is enabled and transcript file in *.srt format is used, if video has transcript.

Import is fixed to support an old style XML format: if video has XML in old format and  `track` node is available,  "Download Transcript" button will be shown and link form track will be used.

@auraz please review.