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

(cherry picked from commit 9596fdc85260eaf29e04a4d8829f4f6fc0f6c5ed)