css: Explode dark theme disabled rule.

This replaces a :disabled selector with individual ones for each
element type; I verified in CSS selector profiling that this change
removes lines that were previously about 10% of profiled CSS selector
processing time for rendering the combined feed.

The reason being that the rendered elements have very few disable-able
elements, but the browser was spending a lot of time trying to match
**every** element for whether it might have been disabled, which was
slow.

This probably reflects a browser bug.

(cherry picked from commit e4567e05ac)

web/styles/dark_theme.css

@@ -342,8 +342,13 @@
         color: inherit;
     }
 
-    /* these are converting grey things to "new grey" */
-    :disabled,
+    /* these are converting grey things to "new grey".
+       :disabled rules are exploded for CSS selector performance reasons. */
+    button:disabled,
+    option:disabled,
+    select:disabled,
+    textarea:disabled,
+    input:disabled,
     input:not([type="radio"]):read-only,
     textarea:read-only,
     #organization-permissions .dropdown-widget-button:disabled,