Flavortown onboarding doesn't trigger for pre-existing MCG accounts

If someone signs up to Flavortown and they already have a Slack account, they won't be automatically added to the Flavortown channels or receive a message in their Slackbot DMs to start the Flavortown Slack onboarding flow.

This behaviour makes sense for full Slack members, but it leaves MCGs in a "soft-locked" state where they don't have a path to upgrade to a full account. (I'm not sure what the upgrade path is for e.g. Campfire, if there is one.)

### Expected behaviour

If new Flavortown participant has an existing Slack account && it's a MCG, send the Slackbot DM and add them to the Flavortown channels.

### Notes

I assume this affects other events as well as Flavortown? But I've only seen it with Flavortown.