Update room filter focus handling to avoid infinite loop

The classes on the search box input were changed without updating the focusing
loop in the room filter which used one of these classes as a boundary condition.
This led to a case that could loop forever.

Regressed by #2267.
Fixes vector-im/riot-web#7926.

src/components/structures/LeftPanel.js

@@ -151,7 +151,7 @@ const LeftPanel = React.createClass({
             }
         } while (element && !(
             classes.contains("mx_RoomTile") ||
-            classes.contains("mx_SearchBox_search")));
+            classes.contains("mx_textinput_search")));
 
         if (element) {
             element.focus();