Allow to use a custom domain per event (#1617)

* Drop support for maindomain_urls/subdomain_urls in plugins

* Allow to use a custom domain per event

* Fix bug when manually saving domains

* Fix custom domains in debugging

* Fix middleware

* Fix middleware again, update docs

src/pretix/multidomain/event_domain_urlconf.py

@@ -0,0 +1,35 @@
+import importlib.util
+
+from django.apps import apps
+from django.conf.urls import include, url
+
+from pretix.multidomain.plugin_handler import plugin_event_urls
+from pretix.presale.urls import event_patterns, locale_patterns
+from pretix.urls import common_patterns
+
+presale_patterns = [
+    url(r'', include((locale_patterns + [
+        url(r'', include(event_patterns)),
+    ], 'presale')))
+]
+
+raw_plugin_patterns = []
+for app in apps.get_app_configs():
+    if hasattr(app, 'PretixPluginMeta'):
+        if importlib.util.find_spec(app.name + '.urls'):
+            urlmod = importlib.import_module(app.name + '.urls')
+            if hasattr(urlmod, 'event_patterns'):
+                patterns = plugin_event_urls(urlmod.event_patterns, plugin=app.name)
+                raw_plugin_patterns.append(
+                    url(r'', include((patterns, app.label)))
+                )
+
+plugin_patterns = [
+    url(r'', include((raw_plugin_patterns, 'plugins')))
+]
+
+# The presale namespace comes last, because it contains a wildcard catch
+urlpatterns = common_patterns + plugin_patterns + presale_patterns
+
+handler404 = 'pretix.base.views.errors.page_not_found'
+handler500 = 'pretix.base.views.errors.server_error'