R1:1f6ad5e7ddd1

Support ".ico" in Celerity and simplify rewite rule configuration

Summary:
See D16734.

- Add ".ico" files to the Celerity map.
- Add a formal route for "/favicon.ico".
- Remove instructions to configure `/rsrc/` and `/favicon.ico` rewrite rules.

Long ago, we served resources directly via `/rsrc/` in at least some cases. As we added more features, this stopped working more and more often (for example, Apache can never serve CSS this way, because it doesn't know how to post-process `{$variables}`).