Fixed wrong bugnote_user_edit_threshold on update

Because there was no check for correct $g_project_override in
bugnote_update.php the default value of "bugnote_user_edit_threshold"
was used instead of one set for the project. This caused problem that
when project had lower user rights setting than the default access
denied exception was thrown when editing bugnote.

Fixes 0030533, PR https://github.com/mantisbt/mantisbt/pull/1818