home: Redirect https://selfhosting.zuliphostname/ to /serverlogin/.

This is a reasonable default page to render for that hostname, and
less confusing than "Organization does not exist."

zerver/tests/test_home.py

@@ -1092,6 +1092,11 @@ class HomeTest(ZulipTestCase):
         self.assertEqual(result.status_code, 302)
         self.assertEqual(result["Location"], "http://testserver")
 
+        with patch("zerver.views.home.get_subdomain", return_value="selfhosting"):
+            result = self._get_home_page()
+        self.assertEqual(result.status_code, 302)
+        self.assertEqual(result["Location"], "/serverlogin/")
+
     def send_test_message(
         self,
         content: str,

zerver/views/home.py

@@ -5,6 +5,7 @@ from typing import List, Optional, Tuple
 from django.conf import settings
 from django.http import HttpRequest, HttpResponse
 from django.shortcuts import redirect, render
+from django.urls import reverse
 from django.utils.cache import patch_cache_control
 
 from zerver.actions.user_settings import do_change_tos_version, do_change_user_setting
@@ -156,6 +157,8 @@ def home(request: HttpRequest) -> HttpResponse:
 
     if subdomain == settings.SOCIAL_AUTH_SUBDOMAIN:
         return redirect(settings.ROOT_DOMAIN_URI)
+    elif subdomain == settings.SELF_HOSTING_MANAGEMENT_SUBDOMAIN:
+        return redirect(reverse("remote_billing_legacy_server_login"))
     realm = get_realm_from_request(request)
     if realm is None:
         return render(request, "zerver/invalid_realm.html", status=404)