register: Make /register redirect to find accounts in root domain without realm.

If there is no realm in the root domain don't show
registration form. Instead just redirect to find
accounts page.

zerver/views/registration.py

@@ -347,7 +347,10 @@ def confirmation_key(request):
 def accounts_home(request, multiuse_object=None):
     # type: (HttpRequest, Optional[MultiuseInvite]) -> HttpResponse
     realm = get_realm(get_subdomain(request))
-    if realm and realm.deactivated:
+
+    if realm is None:
+        return HttpResponseRedirect(reverse('zerver.views.registration.find_account'))
+    if realm.deactivated:
         return redirect_to_deactivation_notice()
 
     from_multiuse_invite = False