dependencies for crunching videos broken

[mbotsch]

decker clean seems to mess up the dependencies for recorded/transcoded videos. After a decker clean all videos are re-transcoded by decker crunch, although the webm-videos did not change and the mp4-videos are still in place.

[mbotsch]

Henrik introduced decker purge as a replacement of decker clean.

Now decker clean will only remove the public directory. This will not cause re-transcoding of videos.
The new decker purge will remove both public and .decker and will cause everything to be re-built, hence also videos to be re-transcoded.