Merge branch 'ee-fix-mr-tabs' into 'master'

Fix MR tabs bottom border (EE)

See merge request gitlab-org/gitlab-ee!5982

app/assets/stylesheets/bootstrap_migration.scss

@@ -177,6 +177,9 @@ table {
 }
 
 .nav-tabs {
+  // Override bootstrap's default border
+  border-bottom: 0;
+
   .nav-link {
     border: 0;
   }