Make CSV import a feature that can be turned on / off.

config/firefly.php

@@ -5,6 +5,7 @@ return [
     'version'                  => '3.4.6',
     'index_periods'            => ['1D', '1W', '1M', '3M', '6M', '1Y', 'custom'],
     'budget_periods'           => ['daily', 'weekly', 'monthly', 'quarterly', 'half-year', 'yearly'],
+    'csv_import_enabled'       => false,
     'piggy_bank_periods'       => [
         'week'    => 'Week',
         'month'   => 'Month',