discourse_sync assumes Discourse username will be Drupal account name, but that's not always the case

Update the MR a bit to pass around what's already on the event. Removed two methods and merged them into one.

Normally, the changes here would be a no-go because they break backwards compatibility, but I highly doubt anyone was calling the sync service themselves manually. Also, this module has no tests, so any change is a shot in the dark, really.

Left some comments on the MR. I think I may have spotted another place where this assumption is being made.