table-sticky-headers: Add z-index CSS property to table headers.

Previously, there were cases where the table contents started to
overlap with the table heading when scrolled down the table.
This was mostly visible in `Custom Emoji` organization UI,
where the images and action table column contents started to
overlap with the table headers.

Rectified the bug by adding a CSS `z-index` property to
`table-sticky-headers` class which takes care of all
such overlapping issues.

Fixes #18906.

static/styles/settings.css

@@ -255,6 +255,7 @@ td .button {
     .table-sticky-headers th {
         position: sticky;
         top: 0;
+        z-index: 1;
     }
 
     .table-striped {