corporate: Revise string method for LicenseLedger objects.

Includes the license counts, date and renewal information as well
as the id field, so that print debugging and displaying these
objects is easier and more useful.

corporate/models.py

@@ -538,6 +538,12 @@ class LicenseLedger(models.Model):
     # equal to the number of activated users in the organization.
     licenses_at_next_renewal = models.IntegerField(null=True)
 
+    @override
+    def __str__(self) -> str:
+        ledger_type = "renewal" if self.is_renewal else "update"
+        ledger_time = self.event_time.strftime("%Y-%m-%d %H:%M")
+        return f"License {ledger_type}, {self.licenses} purchased, {self.licenses_at_next_renewal} next cycle, {ledger_time} (id={self.id})"
+
 
 class SponsoredPlanTypes(Enum):
     # unspecified used for cloud sponsorship requests