CVE-2020-14215: Add migration to clear INVITED_AS_REALM_ADMIN.

This migration fixes any PreregistrationUser objects that might have
been already corrupted to have the administrator role by the buggy
original version of migration 0198_preregistrationuser_invited_as.

Since invitations that create new users as administrators are rare, it
is cleaner to just remove the role from all PreregistrationUser
objects than to filter for just those older invitation objects that
could have been corrupted by the original migration.

zerver/migrations/0261_pregistrationuser_clear_invited_as_admin.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.26 on 2020-06-16 22:26
+from __future__ import unicode_literals
+
+from django.db import migrations
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+def clear_preregistrationuser_invited_as_admin(
+        apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    """This migration fixes any PreregistrationUser objects that might
+    have been already corrupted to have the administrator role by the
+    buggy original version of migration
+    0198_preregistrationuser_invited_as.
+
+    Since invitations that create new users as administrators are
+    rare, it is cleaner to just remove the role from all
+    PreregistrationUser objects than to filter for just those older
+    invitation objects that could have been corrupted by the original
+    migration, which would have been possible using the
+    django_migrations table to check the date when the buggy migration
+    was run.
+    """
+    INVITED_AS_MEMBER = 1
+    INVITED_AS_REALM_ADMIN = 2
+    PreregistrationUser = apps.get_model("zerver", "PreregistrationUser")
+    PreregistrationUser.objects.filter(
+        invited_as=INVITED_AS_REALM_ADMIN).update(
+            invited_as=INVITED_AS_MEMBER)
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0260_missed_message_addresses_from_redis_to_db'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            clear_preregistrationuser_invited_as_admin,
+            reverse_code=migrations.RunPython.noop
+        ),
+    ]