explorer/reducers: don't use FileMetadata

This separates the explorer files state from the fileStorage FileMetadata.
The latter contains extra info that the explorer doesn't need. This
will prevent rerendering each time the file contents change.

src/explorer/newFileWizard/NewFileWizard.tsx

@@ -45,7 +45,7 @@ const NewFileWizard: React.VoidFunctionComponent<NewFileWizardProps> = ({
     const fileNameValidation = validateFileName(
         fileName,
         pythonFileExtension,
-        files.map((f) => f.path),
+        files.map((f) => f.name),
     );
     const [hubType, setHubType] = useState(defaultHub);