[web] kill GMTOFFSET (closes #2154655)
Its last user, set_cookie(expires=...), can just as well interpret its
argument as UTC. Naïve datetime objects with an implicit non-UTC
timezone are just awful.
[web/request] don't play tricks with utc offset
We need an UTC datetime from a time.time()-type value. It turns out
datetime provides a method to do just that. Related to #2154655.