Add sjson and srt to type map.
Merge pull request #3648 from edx/oleg/fix-download-transcript

Add mime-type for srt and sjson when importing course.
Added new error message that displays when HTML5 video is not supported altogether. [BLD-638]
Error message is correctly displayed when HTML5 video is disabled by user
Added error message inside video tag instead. Hide spinner when no HTML5 video is defined or video tag not supported.
Removed exclamation point from error message.
Addressed comments.
Corrected acceptance test.
Added acceptance test on spinner visibility.
Set is_spinner_shown to method instead of property.