Allow viewing history of deleted bugnotes

Because it is not possible to determine the public/private status of a
deleted bugnote, associated history events were only shown to the person
who made the change. This prevented the note's owner and administrators
from seeing other events (e.g. if the admin deletes a note created by
a reporter, the reporter will only see "note added" and the admin "note
deleted".

A note's author is now allowed to see all history events related to that
specific bugnote, even when they occurred due to another user's actions.

Users having private_bugnote_threshold can see all bugnote-related
history events, regardless of whether they authored it or not.

Fixes 0036257