Allow ticket output providers to handle downloads externally (#1402)

* TicketOutput-Providers: Make preview optional; download/attachable optional; optional specific target; update doc

* Spelling fixes in doc

* Changes after code-review

* Changes after code-review

* Commit missing template file

* Allow for redirects instead of files

* Return HTTPResponse with Content-Type text/uri-list on API

* Update API-doc

* Add viewable to spellinglist, fixing doc-test

src/pretix/control/views/orders.py

@@ -16,7 +16,8 @@ from django.db.models import (
 )
 from django.forms import formset_factory
 from django.http import (
-    FileResponse, Http404, HttpResponseNotAllowed, JsonResponse,
+    FileResponse, Http404, HttpResponseNotAllowed, HttpResponseRedirect,
+    JsonResponse,
 )
 from django.shortcuts import get_object_or_404, redirect, render
 from django.urls import reverse
@@ -225,7 +226,8 @@ class OrderDetail(OrderView):
                 'text': provider.download_button_text or 'Ticket',
                 'icon': provider.download_button_icon or 'fa-download',
                 'identifier': provider.identifier,
-                'multi': provider.multi_download_enabled
+                'multi': provider.multi_download_enabled,
+                'javascript_required': provider.javascript_required
             })
         return buttons
 
@@ -340,12 +342,16 @@ class OrderDownload(AsyncAction, OrderView):
                 'message': str(self.get_success_message(value))
             })
         if isinstance(value, CachedTicket):
-            resp = FileResponse(value.file.file, content_type=value.type)
-            resp['Content-Disposition'] = 'attachment; filename="{}-{}-{}-{}{}"'.format(
-                self.request.event.slug.upper(), self.order.code, self.order_position.positionid,
-                self.output.identifier, value.extension
-            )
-            return resp
+            if value.type == 'text/uri-list':
+                resp = HttpResponseRedirect(value.file.file.read())
+                return resp
+            else:
+                resp = FileResponse(value.file.file, content_type=value.type)
+                resp['Content-Disposition'] = 'attachment; filename="{}-{}-{}-{}{}"'.format(
+                    self.request.event.slug.upper(), self.order.code, self.order_position.positionid,
+                    self.output.identifier, value.extension
+                )
+                return resp
         elif isinstance(value, CachedCombinedTicket):
             resp = FileResponse(value.file.file, content_type=value.type)
             resp['Content-Disposition'] = 'attachment; filename="{}-{}-{}{}"'.format(