make sure the room list doesn't grow taller than viewport

by setting the min-height to 0 at every flex level,
since by default it's auto which means grow as tall as you need
to make the content fit.

res/css/structures/_LeftPanel.scss

@@ -51,6 +51,7 @@ limitations under the License.
     overflow-x: hidden;
     display: flex;
     flex-direction: column;
+    min-height: 0;
 }
 
 .mx_LeftPanel .mx_AppTile_mini {