Update settings per vsite, and custom pin domains

Relates to #2306 

* When a vsite does not have a custom domain they can not have a custom pin app
* All sites share the pin app settings from the main site regardless of domain
* when you add a custom pin app for a custom domain vsite, then all file requests use that domain and that site's pin app
* os_fast URL's should work for all pin file downloads
* global private file by pin variable is now available in OS -> Pinserver settings form