Stream-only IA videos must be MPEG4

- https://ddr.densho.org/ddr-densho-1024-37/
- https://archive.org/details/ddr-densho-1024-37
- must have a .mpeg4 extension to work
- extension may be hard-coded in template?
- https://ddr.densho.org/api/0.2/ddr-densho-1024-37/
- or we're selecting the MP4 section from IA metadata
- TODO if ['ia_meta']['files']['mp4']['stream_only'], use filename in  ['ia_meta']['files']['mp4']['original']
