Link to the favicon using the asset helper

This new behavior matches that in `core/server/views/default.hbs`. Specifying the favicon as `/favicon.ico` is wrong because it doesn't obey subdirectory location, plus is just a wrong path.

However, I still get a 404 with the favicon, as the Ghost-core favicon is actually at `<subdir>/shared/favicon.ico`. Changing it to `{{asset "../shared/favicon.ico"}}` works, but feels wrong. I'm expecting #1611 will fix the 404.