Some recent changes to dialogs changed how this works and altered the look of the settings dialogs. This fixes that and makes it so that the header and content scroll independently. Fixes https://github.com/vector-im/riot-web/issues/8329 Fixes https://github.com/vector-im/riot-web/issues/8328
This also brings out some classes that aren't solely applicable to the user settings and will be applicable to the room settings as well.