Fixed header being over issue boards in focus mode

Closes #2473

app/assets/stylesheets/pages/boards.scss

@@ -72,7 +72,7 @@
     height: 100%;
     top: 0;
     background: $white-light;
-    z-index: 100;
+    z-index: 500;
 
     .boards-list {
       height: calc(100vh - 50px);

changelogs/unreleased-ee/issue-boards-focus-mode-fix.yml

+---
+title: Fixed header being over issue boards when in focus mode
+merge_request:
+author: