Combine cache isolation patches and bind to pref

We have two cache isolation patches that we should try to get merged upstream, especially since one of them is a crazy large change to the image cache operation and is likely to generate conflicts in future releases.

However, the patches need work before they can be merged. In addition to the pref, we'll want to make the normal cache isolation built-in, instead of relying on torbutton's stanford-safecache.js and associated observers.

Someone will probably also need to write/updated tests. Bleh.

Getting stuff like this polished and merged is a non-trivial amount of work. At a guess, I'd say around 3 to 4 days worth in total?