clean up video.get_thumb to use Video.extensions

[sawall]

This may actually require some significant refactoring to handle elegantly due to inheritance issues when get_exif_tags is called from album.py.

[sawall]

This includes image.get_thumb and video.get_thumb. I ported them both from settings in Sigal but it's being handled in a horrid way at the moment. Probably utils is the right place to put this sort of thing.

Also, ideally these should use Video.extensions instead of an explicit list of Video extensions.