remote_billing: Implement confirmation flow for legacy servers.

For the last form (with Full Name and ToS consent field), this pretty
shamelessly re-uses and directly renders the
corporate/remote_realm_billing_finalize_login_confirmation.html
template. That's probably good in terms of re-use, but calls for a
clean-up commit that will generalize the name of this template and the
classes/ids in the HTML.

confirmation/models.py

@@ -4,7 +4,7 @@ __revision__ = "$Id: models.py 28 2009-10-22 15:03:02Z jarek.zgoda $"
 import secrets
 from base64 import b32encode
 from datetime import timedelta
-from typing import List, Mapping, Optional, Union
+from typing import List, Mapping, Optional, Union, cast
 from urllib.parse import urljoin
 
 from django.conf import settings
@@ -30,6 +30,9 @@ from zerver.models import (
     UserProfile,
 )
 
+if settings.ZILENCER_ENABLED:
+    from zilencer.models import PreregistrationRemoteServerBillingUser
+
 
 class ConfirmationKeyError(Exception):
     WRONG_LENGTH = 1
@@ -56,7 +59,7 @@ def generate_key() -> str:
     return b32encode(secrets.token_bytes(15)).decode().lower()
 
 
-ConfirmationObjT: TypeAlias = Union[
+NoZilencerConfirmationObjT: TypeAlias = Union[
     MultiuseInvite,
     PreregistrationRealm,
     PreregistrationUser,
@@ -64,6 +67,11 @@ ConfirmationObjT: TypeAlias = Union[
     UserProfile,
     RealmReactivationStatus,
 ]
+ZilencerConfirmationObjT: TypeAlias = Union[
+    NoZilencerConfirmationObjT, "PreregistrationRemoteServerBillingUser"
+]
+
+ConfirmationObjT = Union[NoZilencerConfirmationObjT, ZilencerConfirmationObjT]
 
 
 def get_object_from_key(
@@ -130,6 +138,7 @@ def create_confirmation_link(
     if no_associated_realm_object:
         realm = None
     else:
+        obj = cast(NoZilencerConfirmationObjT, obj)
         assert not isinstance(obj, PreregistrationRealm)
         realm = obj.realm
 
@@ -187,6 +196,7 @@ class Confirmation(models.Model):
     MULTIUSE_INVITE = 6
     REALM_CREATION = 7
     REALM_REACTIVATION = 8
+    REMOTE_SERVER_BILLING_LEGACY_LOGIN = 9
     type = models.PositiveSmallIntegerField()
 
     class Meta:
@@ -223,6 +233,10 @@ _properties = {
     Confirmation.REALM_CREATION: ConfirmationType("get_prereg_key_and_redirect"),
     Confirmation.REALM_REACTIVATION: ConfirmationType("realm_reactivation"),
 }
+if settings.ZILENCER_ENABLED:
+    _properties[Confirmation.REMOTE_SERVER_BILLING_LEGACY_LOGIN] = ConfirmationType(
+        "remote_billing_legacy_server_from_login_confirmation_link"
+    )
 
 
 def one_click_unsubscribe_link(user_profile: UserProfile, email_type: str) -> str: