Support http partial content for videos

[SystemZ]

Currently video files loaded directly after going to their full size url doesn't support rewind etc.
Workaround is F5 after loading to use browser cache set earlier with etag and cache expire headers.

Links:
https://stackoverflow.com/questions/36540610/how-to-serve-http-partial-content-with-go
https://www.sanarias.com/blog/115LearningHTTPcachinginGo