[Feature]: redirect doenet.org/portfolioviewer to legacy site

[dqnykamp]

While searching the web, I found links to potfolioviewer urls like https://doenet.org/publicportfolio/oM0X1zIdsyx7wMKishUWT.

We should redirect those to https://leagcy.doenet.org/publicportfolio/oM0X1zIdsyx7wMKishUWT