feat(management): search users by email in any case

GET /api/v2/users with q=email:"..." answers like Auth0's user search: the
email matches in any letter case, unlike users-by-email, and the answer has
the include_totals shape the Go SDK asks for by default. Any other query is
refused with 400 rather than answered wrong.

Goodfeed's backend checks whether an address is taken with this search
before it lets a user change their email.

cmd/service/service.go

@@ -102,6 +102,7 @@ func main() {
 
 	// Management API endpoints
 	mux.HandleFunc("GET /api/v2/users-by-email", managementHandler.GetUsersByEmail)
+	mux.HandleFunc("GET /api/v2/users", managementHandler.SearchUsers)
 	mux.HandleFunc("POST /api/v2/users", managementHandler.CreateUser)
 	mux.HandleFunc("PATCH /api/v2/users/", managementHandler.UpdateUser)
 	mux.HandleFunc("POST /api/v2/tickets/password-change", managementHandler.PasswordChangeTicket)