If a user already exists with the discord ID we just authenticated, just
return it rather than trying to create another user.
Change-Id: I582c4c8c55f6fb399295b3e691b0bc59e77151f4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4410
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI