Extend matrix.to URL scheme and update spec

There is a current proposal by design to extend the URL scheme for a smoother user experience by adding a client parameter and an inviting-user parameter.

The client parameter will help matrix.to select a sensible default client for the room redirect. It will also help private matrix instance with custom clients pass that information to new users.

The optional inviting-user parameter was intended to provide a visual preview of the inviting user as seen here: https://www.figma.com/file/WSXjCGc1k6FVI093qhlzOP/04-Recieving-share-link?node-id=148%3A341/. However, there is currently no way to do that without opening many forms of phishing attack. I doubt this will make it to the spec.