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/settings.py

@@ -163,6 +163,10 @@ DEFAULTS = {
         'default': 'False',
         'type': bool
     },
+    'ticket_download_nonadm': {
+        'default': 'True',
+        'type': bool
+    },
     'last_order_modification_date': {
         'default': None,
         'type': datetime