Fix fixed width dialogs
Fix fixed width dialogs

Move the max-width to the fixedWidth div rather than the dialog,
otherwise the fixedWidth keeps getting bigger with bigger windows
but the dialog doesn't so it scrolls.

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