Re-enable Alternate Services

Mozilla patched Alternate Services (Alt-Svc) to have first-party isolation: https://bugzilla.mozilla.org/1334690, effective Firefox 54. We disabled Alt-Svc , but in TBB/ESR59 we can potentially re-enable it.

We also need to examine if there are other related headers or mechanisms that could act as supercookie vectors. (Patrick McManus mentioned alt-used as a possibility.) If there are, then those need to be isolated as well.