Style updates: Re-add earth background, change favicon and optimize images

A thought from last week: I like the earth background more in combination with the "everyone" in our message.

Also, 2+ MB in size makes page loading slow which is why I have used imagemagick's convert to optimize the width and quality of images. The favicon is a square screenshot of the background image.

convert earth-1151659.jpg -resize 1920 -quality 80 background.jpg
convert favicon-earth.png -resize 512 favicon.png

I've chosen these generic names as they allow to easily copy/paste a change, becoming the SSoT (single source of truth). If you need to edit the HTML counterparts every time, this may introduce error sources.

I've also moved the originals into a new sub folder, until we figure out how to proceed.

cc @m4r10k @chaosjolo