Validate selection of private template for feedback request (M)

[pieperm]

Depends on #1361 and #1374

Definition of Done

• Validation on front-end to check that the template selected in the URL is permitted
• Display a toast if the template is not allowed, and remove it from the URL
• Validation on back-end to ensure that when making a feedback request, the template that is used is public or, if it is private, it is only accessible to admins and the creator
• Integration tests