Add grayscale image filter

Hat tip to @pheus. Thanks!

I did end up leaving the filter function arguments as the Sass
processor complains when you try calling a filter function without
an argument. :/

netbox/project-static/styles/custom/_misc.scss

@@ -84,6 +84,6 @@ img.plugin-icon {
 body[data-bs-theme=dark] {
   // Assuming icon is black/white line art, invert it and tone down brightness
   img.plugin-icon {
-    filter: invert(100%) brightness(80%);
+    filter: grayscale(100%) invert(100%) brightness(80%);
   }
 }