R1:fbc4967154ba

Improve cache behaviors for font files and other nonstandard resource types

Summary:
Ref T10843. There are actually two separate notions of cacheability here:

- Is this cacheable by the browser (e.g., should we emit "Expires: long in the future")?
- Is this cacheable locally (e.g., should we stick it in APC, or just read it off disk every time)?

These got a little mixed up by D15775, so we aren't currently emitting proper "Expires" headers on font files and a few other resource types.

Straighten…