Isolate HTTP cookies according to first and third party domain contexts

Right now, we've set Tor Browser to block third party cookies. This will probably break some sites. There is a less intrusive option described at https://wiki.mozilla.org/Thirdparty that we should use.

Rebase and test existing patches (originating from https://bugzilla.mozilla.org/show_bug.cgi?id=565965)

Revise requirements according to preliminary tests and devise a broad test plan.

Reimplement and retest to guarantee proper isolation without severely impeding cookie dependent applications.

Document the implementation and optionally a contrast of browser cookie handling.

Pave the way towards a improved privacy panel including a new cookie inspector and API supporting such UI.

Note: This is a metaticket composed of work items in child tickets.