Restrict restSerializer options to the expected values + lean

Part of #2190 (closed)

Follow up to !1481 (merged)

This MR removes validation that was watching for unexpected properties and introduces a hard restriction on the possible options values.

The original option analysis seems to be mostly correct but there is one more option sometimes coming through from bayeux:

{lean: true}

Searching logs for unexpected options:

• ssh -L 5601:localhost:5601 logging-01.prod.gitter (https://gitlab.com/gitlab-com/gl-infra/gitter-infrastructure#production-logging-stats-monitoring)

• search the logs for message: "unexpected options"

• wait for at least 3 days of logs before merging

• after this gets merged, create a MR fixing one low hanging fruit in the user serializer.

Thanks @MadLittleMods for entertaining my urge to clean this up 🍏