Support ordering of plugin nav sections in admin perspective

Currently, the admin navigation does not use extensions for its own items. This means that ordering (insert before / insert after) does not work for plugins contributing top-level items to the admin nav. We should support correct ordering for plugins that contribute these items.

One approach is to switch to the admin nav to use extensions like dev console does. We'll need to investigate whether there is any special logic in admin-nav.tsx that can't be expressed through extensions. An alternative is to keep the static nav and insert items / sections where they belong. Long term, however, we want to migrate admin pages to an extension.

cc christianvogt jhadvig@redhat.com