Fix permission for Observer role in Media Library

[MizukiTemma]

Describe the Bug

Currently users with role "Observer" can replace files in the media library, and a network error occurs when they try to delete a file.

However, they should not be allowed to delete or replace any files. Only uploading shoud be possible for them.

Steps to Reproduce

1. Upload some files as root user.
2. Log out once and log in as observer.
3. Go to Media Library.
4. Try to delete/replace a file.

Expected Behavior

Buttons "Replace file" and "Delete file" are not available for observers, and replacing/deleting requests will be rejected if the user is an observer.

Actual Behavior

The buttons are shown, deleting will be rejected but triggers a network error, replacing will not be rejected.

Additional Information

Add also a warning "You don't have permission to delete or replace files but you can upload files" on the top part of the media library, as in the page tree, poi/event lists.

Traceback