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/components/views/settings/AddPhoneNumber.js

@@ -85,7 +85,6 @@ export default WithMatrixClient(React.createClass({
             Modal.createDialog(ErrorDialog, {
                 title: _t("Error"),
                 description: msg,
-                button: _t("OK"),
             });
         }).finally(() => {
             if (this._unmounted) return;