Allow BaseDialog to take a class for the title <div>

Some dialogs need to set additional classes on the `mx_Dialog_title` `div`
element (for example `danger`).

src/components/views/dialogs/BaseDialog.js

@@ -75,7 +75,7 @@ export default React.createClass({
                 >
                     <TintableSvg src="img/icons-close-button.svg" width="35" height="35" />
                 </AccessibleButton>
-                <div className='mx_Dialog_title'>
+                <div className={'mx_Dialog_title ' + this.props.titleClass}>
                     { this.props.title }
                 </div>
                 { this.props.children }