Fix overlap of error message and sidebar on boards

Changelog: fixed

app/assets/stylesheets/page_bundles/boards.scss

@@ -429,6 +429,10 @@
   height: $input-height;
 }
 
+.issue-boards-content {
+  isolation: isolate;
+}
+
 .issue-boards-content.is-focused {
   position: fixed;
   width: 100%;