emails: Remove pre-email-migration scheduled jobs.

This fixes an issue introduced when we migrated the format of all of
our emails, which caused any old ScheduledJob rows to be corrupted.

zerver/migrations/0079_remove_old_scheduled_jobs.py

@@ -0,0 +1,27 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-05-10 05:59
+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 delete_old_scheduled_jobs(apps, schema_editor):
+    # type: (StateApps, DatabaseSchemaEditor) -> None
+    """Delete any old scheduled jobs, to handle changes in the format of
+    that table.  Ideally, we'd translate the jobs, but it's not really
+    worth the development effort to save a few invitation reminders
+    and day2 followup emails.
+    """
+    ScheduledJob = apps.get_model('zerver', 'ScheduledJob')
+    ScheduledJob.objects.all().delete()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0078_service'),
+    ]
+
+    operations = [
+        migrations.RunPython(delete_old_scheduled_jobs),
+    ]