Same-site cookie support

[classilla]

Many many pieces:

https://bugzilla.mozilla.org/show_bug.cgi?id=795346 (meta bug)

Basic storage and support
https://bugzilla.mozilla.org/show_bug.cgi?id=1286858
https://bugzilla.mozilla.org/show_bug.cgi?id=1452699
https://bugzilla.mozilla.org/show_bug.cgi?id=1286861
Inspector (we will need this to debug it)
https://bugzilla.mozilla.org/show_bug.cgi?id=1298370
Regressions
https://bugzilla.mozilla.org/show_bug.cgi?id=1452496
https://bugzilla.mozilla.org/show_bug.cgi?id=1453814

[classilla]

Another regression: https://bugzilla.mozilla.org/show_bug.cgi?id=1454242

[classilla]

Of questionable utility, given it is sec-low: https://bugzilla.mozilla.org/show_bug.cgi?id=1456652

[classilla]

I'm unable to reproduce 1456652; navigation seems to have changed since, so it's probably not relevant here.