Increased z-index of right sidebar

Fixes issue with active button overlapping it

Closes #15243

app/assets/stylesheets/pages/issuable.scss

@@ -128,6 +128,7 @@
   top: 58px;
   bottom: 0;
   right: 0;
+  z-index: 10;
   transition: width .3s;
   background: $gray-light;
   padding: 10px 20px;