Updates the /api/orgs/create endpoint to:
- not have name / slug be required, will be renamed on first user via
#1870
- support optional quotas
- support optional first admin user email, who will receive an invite to
join the org.
Also supports a new shared secret mechanism, to allow an external
automation to access the /api/orgs/create endpoint (and only that
endpoint thus far) via a shared secret instead of normal login.