make expanding explanation accessible

See also: #4600 LMS-11161 https://openedx.atlassian.net/browse/LMS-11161 
Hi @singingwolfboy.

This would get even more accessible if the following attributes were also be added to the h5:
```aria-expanded="false" role="link"```

```aria-expanded``` value should be toggled upon activation.

Optionally, ```aria-controls="[ID]"``` could also be added with a reference to the ID of the element that gets expanded (currently line 106), but this is above and beyond what is absolutely necessary.  

I'm also not convinced that the change from .click to .keypress targeted at the return key are necessary.  .click is device independent handler and should be triggered by mouse or keyboard action.  I worry about limiting to the return key as on a Mac using Voiceover, links are actually triggered with CTRL + OPT + SPACE.