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/control/forms/event.py

@@ -631,6 +631,11 @@ class TicketSettingsForm(SettingsForm):
         required=False,
         widget=forms.CheckboxInput(attrs={'data-display-dependency': '#id_ticket_download'}),
     )
+    ticket_download_nonadm = forms.BooleanField(
+        label=_("Generate tickets for non-admission products"),
+        required=False,
+        widget=forms.CheckboxInput(attrs={'data-display-dependency': '#id_ticket_download'}),
+    )
 
     def prepare_fields(self):
         # See clean()