Bookmark checkbox can be toggled without being signed in

The bookmark checkbox for exercises (both in the preview and the detail view) can be toggled without being signed in. In the detail view, this increases the bookmark counter locally each time the checkbox is clicked (both when checking it and when unchecking it). Upon a page refresh, these changes disappear.

This can be confusing to users since they might think they are signed in and then wonder why the exercise is not in their bookmarks.

The local bookmark/star count seems to suffer from a race condition, toggling it fast also increases the count when signed in.

Expected behavior

• Bookmark checkbox is deactivated when the user is not signed in (potentially with a message on mouseover)
• Local star count cannot be modified by more than 1 by spamming the checkbox