Log unexpected ffmpeg output
Detect old ffmpeg that can't hide banner
WebM audio option

Fixes #58. Addresses #66.
WebM audio thumbnails
Thumbnails for uploaded WebM videos with audio are overlayed with an image that indicate the video contains sound. Such thumbnails count as composite spoilers.
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.