Audit and enable HTTP/2 push

In #14952 (closed) we plan to enable HTTP/2, but we are postponing enabling HTTP/2 push in case there are potential privacy concerns. Let's investigate any concerns here and hopefully enable push in the future.

added component::applications/tor browser in Legacy / Trac owner::tbb-team in Legacy / Trac priority::medium in Legacy / Trac severity::normal in Legacy / Trac status::new in Legacy / Trac tbb-linkability in Legacy / Trac tbb-performance in Legacy / Trac type::defect in Legacy / Trac labels

Trac:
Keywords: N/A deleted, tbb-performance added

moved from legacy/trac#27127 (moved)

added Bug label and removed 1 deleted label

added Linkability Performance labels and removed 1 deleted label

mentioned in issue #41014 (closed)

added Icebox label

marked this issue as related to #27128 (closed)

added Roadmap::Future label

removed Icebox label

marked this issue as related to #41014 (closed)

changed the description

marked this issue as related to #14952 (closed)

marked this issue as related to tor-browser-build#40627 (closed)

closed