Switch to WEBP

Default image format is now WEBP, in order to reduce images' size.

Also package updates to address this security issue (see also here)

v1.0.0

• Switched from socket.io to WebSocket. (There is a socket-io branch but it relies on Socket.io v2.3.0 which is outdated and presents a security issue)
• cquant doesn't seem to be maintained anymore and is replaced by imagecolors. As a consequence, I had to switch from sharp to gm, although the latter is less effective than the former.
• Optimisation : instead of dealing with the cover of each track, images are cached on album base to reduce processing time.
• Minor graphical changes.