R1:f896dc5392a0

Put a cache in front of Celerity transforms, and update packages

Summary:
Fixes T5094. In some cases we do slightly expensive transformations to resources (inlining images, replacing URIs, building packages). We can throw cache in front of them easily since URIs are already permanently associated with a single resource.

Also browse around and move some CSS/JS into packages.

Test Plan:
Added logging to verify the caches are working, saw moderately improved performance.

Browsed around looking at…