MDL-56820 themes: Fix user search drop down on grading page for boost

Fixes a problem where the drop down list is cut off by overflow.

mod/assign/styles.css

@@ -387,6 +387,7 @@
 }
 
 .path-mod-assign [data-region="configure-filters"] label {
+    display: block;
     padding: 3px 20px;
 }
 

theme/boost/scss/moodle/modules.scss

@@ -500,7 +500,6 @@ div#dock {
 
 .path-mod-assign [data-region="grading-navigation"] {
     padding: 6px;
-    overflow-y: hidden;
 }
 
 .path-mod-assign [data-region="grade-actions"] {