billing: Add Realm.seat_limit.

Currently unenforced, and will mostly need to be set by hand.

zerver/migrations/0191_realm_seat_limit.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.16 on 2018-11-20 04:43
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0190_cleanup_pushdevicetoken'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='realm',
+            name='seat_limit',
+            field=models.PositiveIntegerField(null=True),
+        ),
+    ]

zerver/models.py

@@ -291,6 +291,7 @@ class Realm(models.Model):
     ]
 
     has_seat_based_plan = models.BooleanField(default=False)  # type: bool
+    seat_limit = models.PositiveIntegerField(null=True)  # type: Optional[int]
 
     def authentication_methods_dict(self) -> Dict[str, bool]:
         """Returns the a mapping from authentication flags to their status,