Warn when permissions are overridden

Problem/Motivation

We sometimes override role permissions and it would be useful if a warning could be displayed on /admin/people/permissions.

Steps to reproduce

Create a class implementing ConfigFactoryOverrideInterface that adds some permissions:

  public function loadOverrides($names): array {
    $overrides = [];
    if (in_array('user.role.webmestre', $names)) {
      $overrides['user.role.webmestre']['permissions'][9999] = 'foo';
      $overrides['user.role.webmestre']['permissions'][9998] = 'bar';
    }

    return $overrides;
  }

Proposed resolution

FormOverrides::getFormConfigNames() should handle UserPermissionsForm.