Map schema.org pending properties

[nathansalter]

Map pending properties to pending.schema.org

[nickevansuk]

Just noting that upon further review, fixing forward and using another type of indicator for "pending" properties might be the way to go here.

Steps:

• Update data models to use https://schema.org instead of http://pending.schema.org in sameAs
• Test in .NET to see if that breaks things
• If so add a flag "isPending" or similar to the data model and use this in the .NET generator
• Other generators should be able to simply ignore that flag, and any issues that the above creates can be fixed locally
• This PR may then no longer be required