Favicon

Use [Favicon library](https://github.com/ArthurHoaro/favicon) downloaded from [Packagist](https://packagist.org/packages/arthurhoaro/favicon) internally instead of Google service.

I've also add a cache time limit to refresb favicons monthly.

Bonus point: Improvements have been made in this lib so it has more way to find a favicon than the current service (it doesn't only check the root domain).

Note: Default favicon is now handled by shaarli-api.