Use a fixed source for token images

[Tbaut]

Subsequent to https://github.com/paritytech/fether/pull/451 it isn't possible to load any ressource from an unknown url. This security measure forces us to use a fixed source for token images. I propose to use https://github.com/atomiclabs/cryptocurrency-icons/tree/master/32%402x/color

It could be integrated in our update token script to search for an image in this repo and add it if it's present.