subscription: Add role field to Subscription class.

This commit adds role field to the Subscription class. Currently,
there are two option of roles - STREAM_ADMINISTRATOR and MEMBER.

We also add a property 'is_stream_admin' for checking whether the
user is stream admin or not.

zerver/migrations/0299_subscription_role.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.13 on 2020-06-10 18:44
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0298_fix_realmauditlog_format'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='subscription',
+            name='role',
+            field=models.PositiveSmallIntegerField(db_index=True, default=50),
+        ),
+    ]

zerver/models.py

@@ -2243,6 +2243,11 @@ class Subscription(models.Model):
     # resubscribes.
     active: bool = models.BooleanField(default=True)
 
+    ROLE_STREAM_ADMINISTRATOR = 20
+    ROLE_MEMBER = 50
+
+    role: int = models.PositiveSmallIntegerField(default=ROLE_MEMBER, db_index=True)
+
     # Whether this user had muted this stream.
     is_muted: Optional[bool] = models.BooleanField(null=True, default=False)
 
@@ -2265,6 +2270,10 @@ class Subscription(models.Model):
     def __str__(self) -> str:
         return f"<Subscription: {self.user_profile} -> {self.recipient}>"
 
+    @property
+    def is_stream_admin(self) -> bool:
+        return self.role == Subscription.ROLE_STREAM_ADMINISTRATOR
+
     # Subscription fields included whenever a Subscription object is provided to
     # Zulip clients via the API.  A few details worth noting:
     # * These fields will generally be merged with Stream.API_FIELDS