Disable modal backdrop when !animations setting.

Some users have complained about performance with the backdrop-filter
on .modal-backdrop.show. Partially as a kludge to avoid adding another
toggle, the 'animations' user setting now also disables the backdrop
filter. In practice, this may turn into a more general setting to
remove performance-intensive UI effects.

files/templates/header.html

@@ -299,5 +299,8 @@
 		* {
 			animation: unset !important;
 		}
+		.modal-backdrop.show {
+			backdrop-filter: none !important;
+		}
 	</style>
 {%- endif %}