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;

src/components/views/settings/ChangePassword.js

@@ -70,7 +70,7 @@ module.exports = React.createClass({
 
         var QuestionDialog = sdk.getComponent("dialogs.QuestionDialog");
         Modal.createDialog(QuestionDialog, {
-            title: _t("Warning"),
+            title: _t("Warning!"),
             description:
                 <div>
                     { _t(