Use Google Favicon Service to ensure up to date favicons

[MadaManu]

I have just spotted that some of the favicons are out-dated. Maybe it would be a good idea to use google to get the favicon.

http://www.google.com/s2/favicons?domain_url=http%3A%2F%2Fwww.google.de%2F

Replace the domain_url with the web save version of the domain url and it will get all the favicons perfectly.

[thomasbachem]

I updated the URLs of all broken favicons in d98882b. The problem with using the Google Favicon Service is that most of the time you want to get the favicon from the homepage of a provider, not from the webmail site. Adding that functionality would require us to also store all provider homepages in providers.json. Keeping this ticket open for future consideration.