color picker: Fix colorpicker stream settings.

This color picker did not hide even after exiting stream settings.

Fix by adding logic to auto-close any open color pickers when closing
stream settings.

Tweaked by tabbott to use the existing on-close handler, which is
important if one clicks outside the modal or otherwise navigates
another way.

Fixes #17334.

static/js/subs.js

@@ -743,6 +743,7 @@ export function launch(section) {
             overlay: $("#subscription_overlay"),
             on_close() {
                 browser_history.exit_overlay();
+                $(".colorpicker").spectrum("destroy");
             },
         });
         change_state(section);