MDL-62465 grade: scrolling CSS

Grading pages have their own special javascript to force "fixed" headers
when scrolling a large table. They rely on the entire page being scrolled,
not just the main region.

theme/boost/scss/moodle/grade.scss

@@ -33,6 +33,7 @@
 
 .path-grade-report-grader {
     #region-main {
+        overflow-x: visible;
         min-width: 100%;
         width: auto;
         display: inline-flex;

theme/boost/style/moodle.css

@@ -16175,6 +16175,7 @@ p.arrow_button {
   left: 40vw; }
 
 .path-grade-report-grader #region-main {
+  overflow-x: visible;
   min-width: 100%;
   width: auto;
   display: inline-flex; }