corporate/models: Modify Customer to accommodate self-hosted customers.

This is a part of our efforts to introduce billing for our on-premise
customers.

corporate/models.py

@@ -8,6 +8,7 @@ from django.db import models
 from django.db.models import CASCADE
 
 from zerver.models import Realm, UserProfile
+from zilencer.models import RemoteZulipServer
 
 
 class Customer(models.Model):
@@ -17,7 +18,10 @@ class Customer(models.Model):
     and the active plan, if any.
     """
 
-    realm: Realm = models.OneToOneField(Realm, on_delete=CASCADE)
+    realm: Optional[Realm] = models.OneToOneField(Realm, on_delete=CASCADE, null=True)
+    remote_server: Optional[RemoteZulipServer] = models.OneToOneField(
+        RemoteZulipServer, on_delete=CASCADE, null=True
+    )
     stripe_customer_id: Optional[str] = models.CharField(max_length=255, null=True, unique=True)
     sponsorship_pending: bool = models.BooleanField(default=False)
     # A percentage, like 85.
@@ -30,6 +34,20 @@ class Customer(models.Model):
     # they purchased.
     exempt_from_from_license_number_check: bool = models.BooleanField(default=False)
 
+    @property
+    def is_self_hosted(self) -> bool:
+        is_self_hosted = self.remote_server is not None
+        if is_self_hosted:
+            assert self.realm is None
+        return is_self_hosted
+
+    @property
+    def is_cloud(self) -> bool:
+        is_cloud = self.realm is not None
+        if is_cloud:
+            assert self.remote_server is None
+        return is_cloud
+
     def __str__(self) -> str:
         return f"<Customer {self.realm} {self.stripe_customer_id}>"