make expanding explanation accessible LMS-11161

@talbs 
@singingwolfboy 

Previously, this expanding div wasn't accessible by tab and wouldn't toggle on enter. This address those two issues.

See https://openedx.atlassian.net/browse/LMS-11161 for the ticket
Might be clearer to reuse a named function, instead:
```js
var expandCallback = function(event) {
  event.preventDefault();
  $(this).next('.expandable-area').slideToggle();
  $(this).parent().toggleClass('is-expanded');
}

$('.is-expandable .title-expand').click(expandCallback);
$('.is-expandable .title-expand').keypress(function(e) {
  // only activate on pressing enter
  if (e.which == 13) {
    expandCallback(e);
  }
});
```
Then again, doing it like that is more code. Do you think this is more clear, or less?