Fix default selected namespace when using limited namespace visibility

When the `--namespaces` arg is set to restrict namespace visibility,
the Dashboard was incorrectly defaulting to the first namespace in
the list on page load instead of using the namespace specified in
the URL.

Move the logic to select the default namespace from the top-level
app component into the header where we're already checking for the
selected namespace from the URL.

src/containers/App/App.jsx

@@ -11,7 +11,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import React, { useEffect, useMemo, useState } from 'react';
+import React, { useMemo, useState } from 'react';
 import { useQuery, useQueryClient } from '@tanstack/react-query';
 import { hot } from 'react-hot-loader/root';
 import { Link, Redirect, HashRouter as Router, Switch } from 'react-router-dom';
@@ -181,12 +181,6 @@ export function App({ lang }) {
     isWebSocketConnected
   );
 
-  useEffect(() => {
-    if (!isFetchingConfig && tenantNamespaces.length) {
-      setSelectedNamespace(tenantNamespaces[0]);
-    }
-  }, [isFetchingConfig, JSON.stringify(tenantNamespaces)]);
-
   const logoutButton = <LogoutButton getLogoutURL={() => logoutURL} />;
 
   const namespaceContext = useMemo(

src/containers/HeaderBarContent/HeaderBarContent.jsx

@@ -37,8 +37,10 @@ export default function HeaderBarContent({ isFetchingConfig, logoutButton }) {
   useEffect(() => {
     if (params.namespace) {
       selectNamespace(params.namespace);
+    } else if (tenantNamespaces.length) {
+      selectNamespace(tenantNamespaces[0]);
     }
-  }, [params.namespace]);
+  }, [params.namespace, JSON.stringify(tenantNamespaces)]);
 
   function setPath(path, { dropQueryParams } = {}) {
     navigate(`${path}${dropQueryParams ? '' : location.search}`);

src/containers/NamespacesDropdown/NamespacesDropdown.jsx

@@ -56,7 +56,7 @@ const NamespacesDropdown = ({
     disableWebSocket: true
   });
 
-  let selectedItem = { ...originalSelectedItem };
+  const selectedItem = { ...originalSelectedItem };
   if (selectedItem && selectedItem.id === ALL_NAMESPACES) {
     selectedItem.text = allNamespacesString;
   }