Merge branch 'fix-popover-header-size' into 'master'

Force popover-header to size 14px instead of rems

Closes gitlab-ee#8941

See merge request gitlab-org/gitlab-ce!24065

app/assets/stylesheets/bootstrap_migration.scss

@@ -145,7 +145,8 @@ table {
   pointer-events: none;
 }
 
-.popover {
+.popover,
+.popover-header {
   font-size: 14px;
 }