add rel author and publisher link to cp settings #5058

#5058 

#### Changes

* Add a new CP > Settings section "Structured data" on all CP > Settings forms.
* On all site types, expose a rel=publisher field for a new variable
* On personal sites only, expose a rel=author field for a new variable
* In `vsite_preprocess_html`, if any values exist in this vsite, add head link tags for them.