upgrade: Pass customer_name instead of realm object to upgrade context.

This will help simplify things for remote realms.

corporate/lib/stripe.py

@@ -1637,7 +1637,7 @@ class RealmBillingSession(BillingSession):
     def update_context_initial_upgrade(self, context: Dict[str, Any]) -> None:
         assert self.user is not None
         data = {
-            "realm": self.realm,
+            "customer_name": self.realm.name,
             "email": self.user.delivery_email,
             "is_demo_organization": self.realm.demo_organization_scheduled_deletion_date
             is not None,

templates/corporate/upgrade.html

@@ -9,7 +9,7 @@
 <div id="upgrade-page" class="register-account flex full-page">
     <div class="center-block new-style">
         <div class="pitch">
-            <h1>Upgrade {{ realm.name }} to
+            <h1>Upgrade {{ customer_name }} to
                 {% if free_trial_days %}
                     Zulip Cloud Standard free trial
                 {% else %}