Name presale index + unit test for URL names (#784)

* Name the default URL

If metrics collection is enabled, the index page of the site will fail
to load: without a name, the metrics middleware throws a TypeError.

* Test for names on all URLs

This test passes if all URLs have names. Without names, URLs will cause
the optional metrics middleware to throw a TypeError.

src/pretix/multidomain/maindomain_urlconf.py

@@ -15,7 +15,7 @@ presale_patterns_main = [
     url(r'', include((locale_patterns + [
         url(r'^(?P<organizer>[^/]+)/', include(organizer_patterns)),
         url(r'^(?P<organizer>[^/]+)/(?P<event>[^/]+)/', include(event_patterns)),
-        url(r'^$', TemplateView.as_view(template_name='pretixpresale/index.html'))
+        url(r'^$', TemplateView.as_view(template_name='pretixpresale/index.html'), name="index")
     ], 'presale')))
 ]