Certain Permissions are hard coded to 'Administrator' in CRM_Core_Permission_WordPress

Ran across this with a client with many CiviCRM admins.

Typically, when we end up with a number of CiviCRM administrators we do not make them WP administrators for security reasons. We typically reserve WP admins to a very small subset of accounts and grant permissions in WP by group.

This works fine in WP, however we got a report that the CiviCRM admins cannot see the CMS id when editing contacts. Digging into this the issue is in CRM_Core_Permission_WordPress https://github.com/civicrm/civicrm-core/blob/master/CRM/Core/Permission/WordPress.php#L70

 // for administrators give them all permissions
    if (!function_exists('current_user_can')) {
      return TRUE;
    }
    if (current_user_can('super admin') || current_user_can('administrator')) {
      return TRUE;
    }

We need to change the current_user_can call away from a built in role to testing on a capability. For this use case 'edit_users' makes sense, however we need this a bit more flexible for other use cases. 'manage_options' is not a good choice as that is one of the most removed capabilities.

Would like some feedback on other use cases and then we can submit a PR.

Related: https://issues.civicrm.org/jira/browse/CRM-17587