feat(fxa-settings): implement React pairing pages with Backbone parity
Convert pairing flow from Backbone to React: - Authority pages: AuthAllow, AuthTotp, AuthComplete, AuthWaitForSupp - Supplicant pages: Supp, SuppAllow, SuppWaitForAuth - Pairing channel integration and state machine - Integration models for pairing authority and supplicant reliers - Fix webpack alias for @fxa/shared/l10n module resolution