Portal 5.0 changes

Portals in Blueprint 5.0 are largely the same as v4.x, but they have been tweaked to use the stable React context API instead of the legacy context API.

Instead of the class component getChildContext() API which was expected to return { blueprintPortalClassName: string } to customize the class name applied to all Blueprint 3.x/4.x portals in a given React tree, Blueprint 5.0 now provides a PortalProvider component you may use to define the same custom class. See the v5.0 Portal context options documentation.