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),
+        ),
+    ]