flush metatag cache on settings save #5396

#5396

#### Changes
* When CP > Settings form is submitted, IFF there is a modified value for either "Site title" or "Meta description", then update the vsite node and clear the appropriate metatag cache.
* When a vsite front page is loaded, check the node's metatag cache ID, not the frontpage metatag cache ID.
* Add a helper function (used 3 times) to generate metatag cache ID parts.
* Add validation to prevent users from deleting their site title (use `#required = TRUE`)