Add system check for updated clean URL settings

Overview

Based on my experience and this conversation: https://chat.civicrm.org/civicrm/pl/56au14c7dpb5ddebj69kbfjdir

Clean URLs should be required at this point. It's not as much of a problem in Drupal, but in Wordpress if a site has been upgraded from the old Clean URL settings in civicrm.settings.php and ?civiwp style URLs and civicrm.settings.php has not been updated manually, strange things will being to occur. This largely manifests itself as yellow "Cannot find valid value for ID" error screens that confuse users when registering for an Event or making a Contribution on user-facing forms. This message comes from a tricky session issue and setting clean URLs up has always been the fix.

Current behaviour

Clean URL settings are often not updated by unsuspecting admins as older sites are upgraded to versions of CiviCRM where they should be required.

Proposed behaviour

System check warning appears for Wordpress users if civicrm.settings.php does not include up-to-date clean URL settings.

Comments

This check could be extended to other CMS' now or in the future, which is the motivation behind using its own check class rather than modifying the existing "Cms" for Wordpress checks. I haven't seen this issue with other CMS' so far.

Any thoughts welcome! PR will be provided and linked.