Issue #2102477 by alexpott: Convert remainder of language negotiation settings to configuration system.
Issue #2164025 by skipyT, pwolanin: Improve security of session ID against DB exposure or SQL injection.
Issue #2172561 by Gábor Hojtsy: Config overrides may spill over to undesired places.
Issue #2164367 by alexpott, tim.plunkett, dawehner: Rebuild router as few times as possible per request.