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/base/ticketoutput.py

@@ -54,12 +54,15 @@ class BaseTicketOutput:
 
         If you override this method, make sure that positions that are addons (i.e. ``addon_to``
         is set) are only outputted if the event setting ``ticket_download_addons`` is active.
+        Do the same for positions that are non-admission without ``ticket_download_nonadm`` active.
         """
         with tempfile.TemporaryDirectory() as d:
             with ZipFile(os.path.join(d, 'tmp.zip'), 'w') as zipf:
                 for pos in order.positions.all():
                     if pos.addon_to_id and not self.event.settings.ticket_download_addons:
                         continue
+                    if not pos.item.admission and not self.event.settings.ticket_download_nonadm:
+                        continue
                     fname, __, content = self.generate(pos)
                     zipf.writestr('{}-{}{}'.format(
                         order.code, pos.positionid, os.path.splitext(fname)[1]