Fix multiple loading of javscript for the single problem grader.

This is the cause of issue #2588.

If a gateway quiz has mulitple problems on the same page, then the
single problem grader template is loaded once for each problem.  Each
time the template is loaded, it adds the single problem grader javacript
to the page.  So as many instances of the javascript are active on the
page as there are problems on the page.  As such, the save button event
handlers are added once for each problem on the page as well.  So when
the save button is clicked, the score and comment for that problem are
submitted once for each problem on the page.  All of those requests
firing at the same time can cause authentication to fail because the
session key can be deleted and put back in again by different processes,
and the timing of things can mess things up.  Note that this does reveal
some deeper database issues though.

templates/HTML/SingleProblemGrader/grader.html.ep

@@ -1,8 +1,11 @@
 % use WeBWorK::Utils 'wwRound';
 %
-% content_for js => begin
-	<%= javascript getAssetURL($ce, 'js/ProblemGrader/singleproblemgrader.js'), defer => undef =%>
-% end
+% if (!stash->{jsInserted}) {
+	% stash->{jsInserted} = 1;
+	% content_for js => begin
+		<%= javascript getAssetURL($ce, 'js/ProblemGrader/singleproblemgrader.js'), defer => undef =%>
+	% end
+% }
 %
 <div class="problem-grader">
 	<hr>