v1.1529.0

Remove Flipgrid embed support

Flipgrid no longer exists. URLs like https://flipgrid.com/s/030475b8ceff and the
embedded variant (`?embed=true`) take you to a PDF file [1] telling you about the
retirement of the service.

[1] https://cdn-dynmedia-1.microsoft.com/is/content/microsoftcorp/microsoft/final/en-us/microsoft-product-and-services/microsoft-education/downloadables/flip-ms-edu.pdf