Add a "validated" entry in the full Ledger's Json.

Reviewers:
  @JoelKatz @donovanhide

This is in response to "blocker" RIPD-698.
Yes, that's why the comment is there asking what to do.  So tell me how to do it right!! :-)

There is some logic already in LookupLedger that also calculates whether the ledger is validated.  But that must be wrong also or else this bug would not have been filed.

https://github.com/rec/rippled/blob/ledger-validated/src/ripple/rpc/impl/LookupLedger.cpp#L170-L189