Use DOM ids in get_zephyr

This greatly reduces lag on mouseover selection.

(imported from commit 7d9bc0ebce2aa9781c316033d3e9d21e9da6ee3f)

zephyr/static/js/zephyr.js

@@ -256,10 +256,8 @@ function get_id(zephyr_row) {
     return zephyr_row.attr('zid');
 }
 
-// NB: this is slow (50 ms or more) due to the custom attribute.
-// We instead could compute an id based on the zid and focused table.
 function get_zephyr(zephyr_id) {
-    return $("table.focused_table [zid=" + zephyr_id + "]");
+    return $('#' + (narrowed ? 'zfilt' : 'zhome') + zephyr_id);
 }
 
 function scroll_to_selected() {