Replace List::Utils::first by List::MoreUtils::any

any is more natural, and returns a boolean, which will be useful
in a later commit.
Use Image::Exiftool instead of identify

Avoid calling an external program and parsing its output.
Also, Exiftool supports exif tags in video files, contrary
to identify which is restricted to images.
Add support for videos

viewer: use HTML5 <video> tag
fgallery: videos are encoded in mp4 and webm using ffmpeg. They are
  never included in files/ or the zip archive for download, but
  a "poster" image is generated (from the first second of the video)
  and included as any other image of the gallery.
viewer: add link to parent directory