MDL-62465 CSS: scrolling tables

Tables displayed inside a flex box container need position:relative to
calculate their size or they cause incorrect scrollable regions to appear in the page.

theme/boost/scss/moodle/core.scss

@@ -296,7 +296,9 @@ form.popupform div {
 }
 
 // Minimum height required for a menu in a table inside a scrollable div.
+// Position required for table sizing inside a flex container.
 .no-overflow > .generaltable {
+    position: relative;
     margin-bottom: 0;
     min-height: 10em;
 }

theme/boost/style/moodle.css

@@ -9004,6 +9004,7 @@ form.popupform div {
   overflow: auto; }
 
 .no-overflow > .generaltable {
+  position: relative;
   margin-bottom: 0;
   min-height: 10em; }