Fall back to flash if source list is not compatible with browser

Occasionally we have encoding issues, whereby a html5 enabled browser such as FireFox cannot play a video if their supported source mime types such as `video/webm` are missing or corrupted within our source list. This is due to issues in the multimedia transcoding pipeline and a race-condition in the editing tools. 

In these instances we want to attempt to play the `video/mp4` via the flash player instead. Unfortunately, VideoJS doesn't currently do this out of the box. Therefore, this – quite hacky– attempts to solve this problem in the interim until we have native functionality via VideoJS. I have started a separate thread on their repo to discuss this and offer to carry out the work.

/cc @rich-nguyen @ironsidevsquincy @mattosborn