Fix #456 -- Allow products to be excluded from ticket-generation (#483)

* Added non-admission setting to event

`ticket_download_nonadm` now setting in storage. Still need logic for
order page/PDF generation.
Works on pretix/pretix#456.

* Download button considers `ticket_download_nonadm`

Modified Django tags to look at item admission attribute and
`ticket_download_nonadm` setting.
Works on pretix/pretix#456.

* Ticket output for non-admission disabled

PDFs/etc. will only be permitted/generated for items with the
`admission` attribute, or if the `ticket_download_nonadm` event setting
is true. Applies to single and whole-order ticket downloads.
Works on pretix/pretix#456.

* Fixed product exclusion in PDF output

Forgot PDF output was a plugin, now includes same check as base
`BaseTicketOutput.generate_order`.
Works on pretix/pretix#456

src/pretix/plugins/ticketoutputpdf/ticketoutput.py

@@ -93,6 +93,8 @@ class PdfTicketOutput(BaseTicketOutput):
         for op in order.positions.all():
             if op.addon_to_id and not self.event.settings.ticket_download_addons:
                 continue
+            if not op.item.admission and not self.event.settings.ticket_download_nonadm:
+                continue
             self._draw_page(p, op, order)
         p.save()
         outbuffer = self._render_with_background(buffer)