Retrieve Web Proposal for a design.

A Web Proposal is a customer facing proposal that can be given to homeowners to view proposals generated using Aurora Sales Mode. A Web Proposal is only accessible through a valid url which expires after a certain amount of time (To get more information on the Web Proposal expiration time, please contact your Aurora account team). A Web Proposal can only have a single non-expired url at a time. If a Web Proposal has not been created yet then the url will be "null" & expired will be "false".