Forced anonymity hours

Allows a number of hours per day (distributed semi-randomly) to have forced global anonimity. No names or tripcodes are written to the database at the time.
Simplified and integrated into composite spoilers WebM audio thumbnail code.
Moved assets to separate repo. Added kana symlink for static webserving.
Log unexpected ffmpeg output
Detect old ffmpeg that can't hide banner
WebM audio option