Change onboarding workflow

[rickr]

When a user joins. They should:

• Be sent a welcome message explaining how mentorship and squads work
• Be able to respond with 'yes join', or 'no'
• If yes:
• [ ] be invited to the least populated squad
• [ ] have their info added to the mentorship airtable
• [ ] prompt them to fill in the mentorship airtable form

[rickr]

Workflow has changed. Closing this out for now.