message: Add `key` prop to RTE decorated entities (Pills and links)
  This will cause a re-render when the URL of the pill/link changes.
  fixes vector-im/riot-web#4718
diff: @@ -190,11 +190,11 @@ export default class MessageComposerInput extends React.Component {
  const Pill = sdk.getComponent('elements.Pill');
  const {url} = Entity.get(entityProps.entityKey).getData();
  if (Pill.isPillUrl(url)) {
  - return <Pill url={url} room={this.props.room} offsetKey={entityProps.offsetKey}/>;
  + return <Pill url={url} key={url} room={this.props.room} offsetKey={entityProps.offsetKey}/>;
  }
  return (
  - <a href={url} data-offset-key={entityProps.offsetKey}>
  + <a href={url} key={url} data-offset-key={entityProps.offsetKey}>
  {entityProps.children}
  </a>
  );
  