Fix settings direct chat
Fix settings direct chat

We were expecting this function to return a string user ID, but it
returned a user object

Make it return a user ID and add jsdoc to say what it returns.

Fixes https://github.com/vector-im/riot-web/issues/8180

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk