Use file_validators instead of Papeclip validations

Since we're going to remove Paperclip and Active Storage doesn't provide
any validations, we have to either write our own validation rules or use
a different gem.

We're using the file_validators gem instead of the
`active_storage_validations` gem because the latter doesn't support
proc/lambda objects in size and content type definitions. We need to use
them because in our case these values depend on settings stored in the
database.

config/locales/es/general.yml

@@ -150,6 +150,7 @@ es:
         submit_button: Guardar cambios
   errors:
     messages:
+      allowed_file_content_types: "el tipo de contenido debe ser uno de los siguientes: %{types}"
       user_not_found: Usuario no encontrado
       invalid_date_range: "El rango de fechas no es válido"
   form: