emails: Replace string_id from sponsorship request email context.

Use of `string_id` in the sponsorship request email content was
removed in commit d3834f8b9, but it is still used in the email
subject.

Updates the email subject to use the billing_entity_display_name,
which is still the Realm.string_id for Zulip Cloud organizations.

Sets this string as "billing_entity" in the context and subject
template.

corporate/lib/stripe.py

@@ -2245,8 +2245,7 @@ class BillingSession(ABC):
         context = {
             "requested_by": user_info["name"],
             "user_role": user_info["role"],
-            # TODO: realm_string_id needs to be replaced by something more generic.
-            "string_id": request_context["realm_string_id"],
+            "billing_entity": self.billing_entity_display_name,
             "support_url": support_url,
             "organization_type": org_type_display_name,
             "website": sponsorship_request.org_website,

templates/zerver/emails/sponsorship_request.subject.txt

@@ -1 +1 @@
-Sponsorship request ({{ organization_type }}) for {{ string_id }}
+Sponsorship request ({{ organization_type }}) for {{ billing_entity }}