Answer checks should offer feedback to assistive tech. This commit adds

a page level javascript SR object to enable reading of alert messages.
LMS-2158

lms/templates/problem.html

@@ -7,7 +7,7 @@
 
 <div class="problem-progress"></div>
 
-<div class="problem">
+<div class="problem" role="application">
   ${ problem['html'] }
 
   <div class="action">
@@ -23,7 +23,7 @@
     <input class="save" type="button" value="${_('Save')}" />
     % endif
     % if answer_available:
-    <button class="show"><span class="show-label">${_('Show Answer(s)')}</span> <span class="sr">${_("(for question(s) above - adjacent to each field)")}</span></button>
+    <button class="show"><span class="show-label" aria-hidden="true">${_('Show Answer')}</span> <span class="sr">${_("Reveal Answer")}</span></button>
     % endif
     % if attempts_allowed :
     <div class="submission_feedback">