This commit adds support for an optional `prompt` parameter in the
payload of the /session/sso_provider endpoint. If an SSO Consumer
adds a `prompt=none` parameter to the encoded/signed `sso` payload,
then Discourse will avoid trying to login a not-logged-in user:
* If the user is already logged in, Discourse will immediately
redirect back to the Consumer with the user's credentials in a
signed payload, as usual.
* If the user is not logged in, Discourse will immediately redirect
back to the Consumer with a signed payload bearing the parameter
`failed=true`.
This allows the SSO Consumer to simply test whether or not a user is
logged in, without forcing the user to try to log in. This is useful
when the SSO Consumer allows both anonymous and authenticated access.
(E.g., users that are already logged-in to Discourse can be seamlessly
logged-in to the Consumer site, and anonymous users can remain
anonymous until they explicitly ask to log in.)
This feature is similar to the `prompt=none` functionality in an
OpenID Connect Authentication Request; see
https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest
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