Replace "How it works" picture by an improved one

Live preview: http://bitcoinhowitworks.zapto.org/en/how-it-works

This pull request replaces the "How it works" picture by a different one with the following improvements:

Show a concrete real use case between a mobile and a tablet
Stop displaying Bitcoin addresses
Drop user's shadows that remind hackers / bad stuff icons a bit too much
Give a better idea of what the decentralized network can look like
Use a "sync" icon to illustrate a consensus on the network
Remove translatable texts from images to make them easier to maintain
Adapt image size and content for mobiles