Moved public folder to frontend

- The files in the public folder are all specific to the frontend:
  - ghost.css / ghost.min.css contains styles for default templates like unsubscribe, private and error
  - favicon is the default for the frontend
  - robots.txt is the default robots.txt for the frontend
  - sitemap.xsl is the stylesheet for the sitemap
- Therefore, they should live in the frontend!

core/shared/config/overrides.json

@@ -10,7 +10,7 @@
         "internalAppPath": "core/frontend/apps/",
         "internalAdaptersPath": "core/server/adapters/",
         "migrationPath": "core/server/data/migrations",
-        "publicFilePath": "core/server/public"
+        "publicFilePath": "core/frontend/public"
     },
     "apps": {
         "internal": [