Final touches to ProjectFiles component

[dotNomad]

This PR:

• makes the Project Files TreeSection expanded by default
• adds tooltips similar to the precursor Project Files view
• adds relDir to the /files API endpoints that give us the relative directory for the file
• Uses the new relDir to add directory path descriptions to the file items
  • We cannot use the method before since we cannot use Node's path.dirname()

[sagerb]

LGTM