Add files for the new favicon system

Use https://github.com/ArthurHoaro/favicon
See https://github.com/FreshRSS/FreshRSS/issues/290
Change favicon getter behaviour

- Use Favicon library
- Fix coding style
- Change default favicon

Fix https://github.com/FreshRSS/FreshRSS/issues/290
Fix typo coding style

See https://github.com/FreshRSS/FreshRSS/issues/290
Merge branch '290-favicons' into dev