Revert RoutingContext#user() returning UserContext to User #1176

Job	Run time
IT / GraphQLWS-Tests (18.13.0)	58s
IT / SockJS-WriteHandler-Tests (16.13.1)	1m 30s
CI (ubuntu-latest, 21) / Run tests	5m 50s
CI (windows-latest, 11) / Run tests	6m 48s
CI (ubuntu-latest, 11) / Run tests	5m 58s
Deploy	0s
	21m 4s

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