Allow users with the Organizers role to sign in via a Discord Oauth2
handshake, creating a user in the users table and adding the ID of that
user to the session.
Change-Id: I39d9e17433e71b07314b9eabb787fb9214289772
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4409
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>