Give mx_RoomView_messageListWrapper its `min-height: 100%` back

because the spinner now exists separately to it
(as mx_RoomView_messagePanelSpinner) so their CSS is disentangled.

src/skins/vector/css/matrix-react-sdk/structures/_RoomView.scss

@@ -118,6 +118,8 @@ limitations under the License.
     max-width: 960px;
     margin: auto;
 
+    min-height: 100%;
+
     display: flex;
 
     flex-direction: column;