Update admin_menu so it doesn't use hook_page_alter().

first attempt at a PR for https://github.com/backdrop/backdrop-issues/issues/294
I'm not 100% sure about this approach. Some of these things shouldn't need to be specified or could be a hassle to maintain, e.g. "version" shouldn't be hard-coded here. "group" should use one of the JS constants for group names instead of the explicit value.