[#34] Add key to react list

The list cached the height while switching between applications.
We fix this with the use of the key property, that means that react will
not reuse the component and make a new one.

see https://reactjs.org/docs/reconciliation.html

ui/src/pages/Messages.js

@@ -82,7 +82,8 @@ class Messages extends Component {
                 {hasMessages
                     ? (
                         <div style={{width: '100%'}}>
-                            <ReactList ref={(el) => this.list = el}
+                            <ReactList key={appId}
+                                       ref={(el) => this.list = el}
                                        itemRenderer={this.renderMessage}
                                        length={messages.length}
                                        threshold={1000}