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.
@Srokap you added the X-Real-IP check, you might want to verify this logic.