Adds a Discourse ID authenticator. Not available for use in production
just yet, but soon communities will be able to use this service to let
users authenticate using a central Discourse ID account.
Includes a support for a `/revoke` action, allowing users to log out of
multiple client instances from a central auth service.
Internal ticket: t/155397
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Co-authored-by: Loïc Guitaut <loic@discourse.org>
