R1:65a2b5e21934

Route hard-coded "/favicon.ico" requests to a favicon resource

Summary:
See PHI1719. User agents making hard-coded requests to "/favicon.ico" currently 404. This is a mild source of log noise, and we can reasonably route this request.

Limitations:

- This only routes the "PlatformSite". Other sites (custom Phame blogs, third-party sites, Phurl redirectors) won't route here for now.
- This returns a "Location:" redirect to the correct resource rather than icon data directly. This produces the right…