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/base/services/tickets.py

@@ -47,6 +47,9 @@ def generate_order(order: int, provider: str):
             prov = response(order.event)
             if prov.identifier == provider:
                 filename, ttype, data = prov.generate_order(order)
+                if ttype == 'text/uri-list':
+                    continue
+
                 path, ext = os.path.splitext(filename)
                 for ct in CachedCombinedTicket.objects.filter(order=order, provider=provider):
                     ct.delete()
@@ -124,6 +127,9 @@ def get_tickets_for_order(order, base_position=None):
         if not p.is_enabled:
             continue
 
+        if p.download_handled_by_frontend:
+            continue
+
         if p.multi_download_enabled and not base_position:
             try:
                 if len(positions) == 0: