Put back default strings on dialogs

But make them work by calling _t in render rather than
getDefaultProps().

Also sort out some 'Warning!' strings

src/createRoom.js

@@ -107,7 +107,6 @@ function createRoom(opts) {
         Modal.createDialog(ErrorDialog, {
             title: _t("Failure to create room"),
             description: _t("Server may be unavailable, overloaded, or you hit a bug") + ".",
-            button: _t("OK"),
         });
         return null;
     });