Don't send entity fields using query parameters on admin edit pages

[GabrielMajeri]

I've seen that the admin interface uses query parameters in the edit page href to store and pass the fields of the existing entity. This is not okay for several reasons:

• It's inefficient and URLs have length/character limitations.
• It's a potential security issue (we don't want secret keys and such being stored in the less-secured page URL).
• It's weird for the user to see such internal information in the page's URL.

How this should work instead is:

• Use a Next.js route parameter to pass in the entity ID.
• Look it up in the database; render a 404 error if it doesn't exist anymore.
• Check if the user has permissions to edit/view that entity.
• Use the data loaded from the database as initial values for the form fields.