Fix JSON export button with navigation by UUID enabled

When using UUID-based navigation in Web UI, JSON export button is hidden and cannot be used to export Document metadata.

Steps to reproduce:

1. Enable UUID-based navigation in Web UI using the following contribution OR navigate to a document using URL http://localhost:8080/nuxeo/ui/#!/doc/DOCUMENT_UUID_HERE

<extension target="org.nuxeo.runtime.ConfigurationService" point="configuration">
  <property name="org.nuxeo.web.ui.router.key.document">uid</property>
</extension>

2. Navigate to any document and open browser extension.

Expected behavior: JSON Export button is visible and able to be clicked.

Actual behavior: JSON Export is not visible.