Refactor and internationalize `IssueEventListItem`

This component could benefit from a refactoring similar to EventsScreen, where every event type would be handled by a dedicated method.

Instead of the big switch/case in render(), we could have something like this:

render() {
    const { repository, event } = this.props;
    const handler = camelCase(`handle_${event.event}`); // lodash's camelCase

    if (typeof this[handler] === 'function') {
      return this[handler](event, repository);
    }

    return null;
}

and implement handleReviewRequested(), handleLabeled(), etc.

We would then wrap strings with utils.t() to internationalize this component.