MDL-55782 groups: wrap long group names (theme: boost)

In the groups UI if a group name is very long it now no longer underlaps
the user column on its right hand side.

theme/boost/scss/moodle/user.scss

@@ -287,6 +287,7 @@
 
         select {
             min-width: 175px;
+            max-width: 90%;
         }
     }
 }