Fix 0014340: Reporters can use SOAP to update bugnotes without permission

The access checks inside bugnote_update.php and
api/soap/mc_issue_api.php differed. Users were incorrectly allowed via
the SOAP interface to update the bugnotes of other users. Instead of
comparing the SOAP user's access level to $g_update_bugnote_threshold,
$g_add_bugnote_threshold was used instead.

This posed a problem because the default installed state of MantisBT is
to allow the REPORTER access level to submit bugs via the SOAP API. Thus
in the default installed state, any user who could submit a bug could
also update/modify the bugnotes of any other user.

Access checks within bugnote_update.php and api/soap/mc_issue_api.php
should now be equivalent.

Thanks to Roland Becker and Damien Regard (both MantisBT developers) for
finding and reporting this problem.