Preview a contract before sending it

Takes exactly the body of a contract request and returns a link to the
document that request would send, as it stands before the signers act:
every variable and pre-filled value is filled in. Nobody is notified and
no contract is created. The request is checked exactly as a send is,
so this returns the same errors a send would. The link opens with no
headers and works for one hour; ask for a new preview after that.