message: Fix `sticky-discussion-sidebar` z-index conflict with repo header
diff: .rgh-sticky-sidebar {
  position: sticky;
  top: 60px !important; /* Matches sticky header's height */
  - z-index: 26 !important;
  + z-index: 31 !important;
  }
  /* Hides the last divider (on pull requests) to avoid https://user-images.githubusercontent.com/10238474/62282128-af6fb980-b457-11e9-8b18-c29687b88da1.gif */
  