send_email: Add ScheduledEmail support for multiple recipients.

Follow up on 92dc363. This modifies the ScheduledEmail model
and send_future_email to properly support multiple recipients.

Tweaked by tabbott to add some useful explanatory comments and fix
issues with the migration.

zerver/migrations/0211_add_users_field_to_scheduled_email.py

@@ -0,0 +1,36 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.20 on 2019-03-14 01:11
+from __future__ import unicode_literals
+
+from django.conf import settings
+from django.db import migrations, models
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+def set_users_for_existing_scheduledemails(
+        apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    ScheduledEmail = apps.get_model("zerver", "ScheduledEmail")
+    for email in ScheduledEmail.objects.all():
+        if email.user is not None:
+            email.users.add(email.user)
+        email.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0210_stream_first_message_id'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='scheduledemail',
+            name='users',
+            field=models.ManyToManyField(to=settings.AUTH_USER_MODEL),
+        ),
+        migrations.RunPython(set_users_for_existing_scheduledemails, reverse_code=migrations.RunPython.noop),
+        migrations.RemoveField(
+            model_name='scheduledemail',
+            name='user',
+        ),
+    ]