[Bug] Do not overwrite the content of JSONTextField while the user is editing such (#895)

* Do not overwrite the context of JSONTextField while the user is editing such

* Style changes

* Handle case where valid JSON could potentially be overwritten.

js/panel-config/json_textarea.js

@@ -32,6 +32,7 @@ export default class JSONTextArea extends Component {
   }
 
   componentWillReceiveProps({ value }) {
+    if (value === this.props.value) return;
     this.setState({
       value: JSON.stringify(value, null, 2),
       isValid: true,