chore(http): Uses Symfony HttpFoundation instead of our PHP 5.2 port

Biggest change here is that ElggSession now uses a Symfony session for
underlying storage instead of a “storage” object. This is just because,
since we ported this, Symfony moved its getters/setters from the storage
object to the higher level session.
FYI I've submitted a [docs fix](https://github.com/symfony/symfony-docs/pull/4710) to Symfony so this isn't such a mystery.