Add favicon

[adamnfish]

Adds favicons optimized for various devices.

[elrido]

I merged your commit into the fork, optimizing the images a bit. Thanks!

[rugk]

Did you used https://realfavicongenerator.net/? There you can optimize the favicon's very nicely.
If you already did I think it would be a good idea to add the JSON for the API so the icon creation process can be reproduced.