Use favicon.php for /favicon.ico of all wikis

This allows legacy clients to see the same favicon as the one sent by
MediaWiki due to $wgFavicon. Also, using a script for configurable
favicons removes one of the very few remaining reasons for keeping the
document roots for second-level subdomains separate.

Also, removed some obsolete yaseo references, and removed obsolete
mod_mmap_static configuration. Both favicon.ico and robots.txt are
scripts now.

Change-Id: I35d3af43b9bbf0fd4bcdf42e35c31c74a76445aa