Simplify settings dialog CSS

Stop the settings dialogs from requiring special styles on the
mx_Dialog which required passing in a classname from anywhere the
settings dialogs were opened (although this still requires
static=true). Some of the things have now been adopted for all dialogs
(border-radius), others have been moved to within the dialog content.

src/stores/RoomViewStore.js

@@ -120,7 +120,7 @@ class RoomViewStore extends Store {
                 const RoomSettingsDialog = sdk.getComponent("dialogs.RoomSettingsDialog");
                 Modal.createTrackedDialog('Room settings', '', RoomSettingsDialog, {
                     roomId: payload.room_id || this._state.roomId,
-                }, 'mx_SettingsDialog', /*isPriority=*/false, /*isStatic=*/true);
+                }, /*className*/null, /*isPriority=*/false, /*isStatic=*/true);
                 break;
             }
         }