Adds a link to the scp/tickets.php page (Tickets) allowing for the export of
the data to CSV file. This is preliminary and still needs a few items:
  * An icon + button CSS definition
  * Support for alternate columns (staff assigned, etc.)

Also add beginnings of complex export support (PDF, JSON  and other
formats
to follow). CSV is fully supported for now.

And for brevity, add a Http::download method to allow for consistent and
browser-independent forced page content downloads

Add a %ticket_history table to track ticket states over time, correlated to
the user account changing the ticket. Then, tickets are tracked to the table
when (re)opened, closed, overdue, transferred and assigned.

Change href definitions for embedded links in the ticket thread so that the
click will be bounced through a script. This will hide the original referrer
URL where the link originated.

Also add a cron job to perform the task on demand
Change the implementation for Ticket::deleteAttachments to call this method
after simply removing all entries from the %ticket_attachment table

This will include other information described in email filters that mark the
email as rejected. For instance, sender-name, message body content, etc. It
also allows for other matching methods (contains, !=, !contains), rather
than the quick banlist method from EmailFilter::isBanned().

Also fix bug in Filter::__construct that prevented any filter from ever
matching