disable "Add overlay" button or improve error message `403` when number of custom layout is greater equal `CUSTOM_LAYOUTS`

A user who has reached the limit of CUSTOM_LAYOUTS overlays can click on the "Add overlay" button, when clicking "UPLOAD" receives an error popup with the text

Unexpected error occurred:
http://127.0.0.1:8080/minerva//api//files/ rejected with status code: 403