Our HTTP disk cache is currently manually tested against various sites.
This patch adds some tests to cover various scenarios, including non-
cacheable responses, expired responses, and revalidation.
In order to ensure we hit the disk cache in RequestServer, we must
disable the in-memory cache in WebContent.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
