Added user-create API endpoint

- Required extracting logic into repo.
- Changed some existing creation paths to standardise behaviour.
- Added test to cover new endpoint.
- Added extra test for user delete to test migration.
- Changed how permission errors are thrown to ensure the right status
  code can be reported when handled in API.

app/Exceptions/Handler.php

@@ -101,6 +101,10 @@ class Handler extends ExceptionHandler
             $code = $e->status;
         }
 
+        if (method_exists($e, 'getStatus')) {
+            $code = $e->getStatus();
+        }
+
         $responseData['error']['code'] = $code;
 
         return new JsonResponse($responseData, $code, $headers);