The three sites that refuse to create or rename an operator onto a
customer's email (Admin\Settings::saveAccount(), ::inviteStaff(), and
clupilot:create-operator) all checked Customer::where('email', ...) as
a proxy for "does this address already have a portal login". A users
row with no matching customers row — an email changed on one side
only, or legacy/orphaned data — passed straight through: this dev
database already had one.
Extracted the three copies into Customer::emailTaken(), which checks
both tables directly, so the three sites cannot drift from each other
again.
58835a1051