Feature/optional team provisioning (#1487)

* Event creation: Add option to select existing team instead of provisioning.

* Event creation: disallow event team provisioning on organizer level.

* Event team provisioning: update default setting location and display strings.

* Update src/pretix/control/views/main.py

Co-Authored-By: Raphael Michel <mail@raphaelmichel.de>

src/pretix/base/settings.py

@@ -683,6 +683,10 @@ Your {event} team"""))
         'default': '',
         'type': LazyI18nString
     },
+    'event_team_provisioning': {
+        'default': 'True',
+        'type': bool
+    },
     'update_check_ack': {
         'default': 'False',
         'type': bool