Form should be a POST not a GET

Using GET in Bug Monitor Add form on view.php, may cause bugs viewed by
user to be auto-monitored because browsers/extensions may pre-load any
GET URL, including from forms; GET is specified as read-only.

Fixes 0030922, PR https://github.com/mantisbt/mantisbt/pull/1842

Signed-off-by: Damien Regad dregad@mantisbt.org

Changes to original submission: improved commit message