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/dialogs/InteractiveAuthDialog.js

@@ -20,6 +20,7 @@ import Matrix from 'matrix-js-sdk';
 import React from 'react';
 
 import sdk from '../../../index';
+import { _t } from '../../../languageHandler';
 
 import AccessibleButton from '../elements/AccessibleButton';
 
@@ -99,7 +100,7 @@ export default React.createClass({
         return (
             <BaseDialog className="mx_InteractiveAuthDialog"
                 onFinished={this.props.onFinished}
-                title={this.state.authError ? 'Error' : this.props.title}
+                title={this.state.authError ? 'Error' : (this.props.title || _t('Authentication'))}
             >
                 {content}
             </BaseDialog>