fix(shared): prevent global keydown

This fixes a regression introduced via a6dfc16.

The prior change switched to `onKeyDown`, which is an synthetic
event handler that cannot prevent default (on global clicks).

What we need to prevent default (global) clicks is actual DOM event
handling. Quoting from the Inferno documentation [1]:

> Lower case events will bypass Inferno's event system in favour of
> using the native event system supplied by the browser.

[1]: https://www.infernojs.org/docs/guides/event-handling

packages/dmn-js-shared/src/components/ContentEditable.js

@@ -143,7 +143,7 @@ export default class ContentEditable extends Component {
     }
   };
 
-  onKeydown = (event) => {
+  onkeydown = (event) => {
 
     // enter
     if (event.which === 13) {
@@ -182,7 +182,7 @@ export default class ContentEditable extends Component {
   };
 
   // TODO(barmac): remove once we drop IE 11 support
-  onKeyPress = (event) => {
+  onkeypress = (event) => {
     if (this.onInputIEPolyfill) {
       this.onInputIEPolyfill(event);
     }
@@ -243,11 +243,11 @@ export default class ContentEditable extends Component {
         spellCheck="false"
         data-placeholder={ placeholder || '' }
         onInput={ this.onInput }
-        onKeyPress={ this.onKeypress }
+        onkeypress={ this.onkeypress }
         onPaste={ this.onPaste }
         onFocus={ this.onFocus }
         onBlur={ this.onBlur }
-        onKeyDown={ this.onKeydown }
+        onkeydown={ this.onkeydown }
         ref={ node => this.node = node }
         dangerouslySetInnerHTML={ { __html: value } }></div>
     );