make expanding explanation accessible

See also: #4600 LMS-11161 https://openedx.atlassian.net/browse/LMS-11161 
@talbs I'm OK with the heading.  I'm also OK with adding the keypress handler.  @adampalay's comment sparked my curiosity so I did some more testing.  With AT on, hitting return worked just fine, but without it, I could not activate the link with the return key.  So, I think if we add the attributes and behavior I suggested in https://github.com/edx/edx-platform/pull/4602/files#r15582302 we should be good to go on this.