This commit adds three new settings that allow the operator to
pre-define the `queue`, `priority` and `due_date` fields for public
tickets. If one of these settings are present the corresponding input
field is hidden from the form.
The settings are the following:
HELPDESK_PUBLIC_TICKET_QUEUE = 'website'
HELPDESK_PUBLIC_TICKET_PRIORITY = 2
HELPDESK_PUBLIC_TICKET_DUE_DATE = ''
If the due date is set to the empty string, no due date is saved in the
ticket (analogously to the form). The other settings should be
self-explanatory.