clear namespaces on logout and properly render nested namespaces in the namepace picker (#7186)

* reset namespace cache when a user logs out

* fix issue where if you log in to a namespace with an initial /, nested namespaces would not show up in the navigation

* set an empty list of namespaces instead of ignoring error

* add tests for namespace bugs

* use this consistently in template

ui/app/components/namespace-picker.js

@@ -126,6 +126,7 @@ export default Component.extend({
   // gets set as  'lastMenuLeaves' in the ember concurrency task above
   menuLeaves: computed('namespacePath', 'namespaceTree', function() {
     let ns = this.get('namespacePath');
+    ns = ns.replace(/^\//, '');
     let leaves = ancestorKeysForKey(ns);
     leaves.push(ns);
     leaves = this.maybeAddRoot(leaves);