retention: Use new ArchiveTransaction model.

We add a new model, ArchiveTransaction, to tie archived objects together
in a coherent way, according to the batches in which they are archived.
This enables making a better system for restoring from archive, and it
seems just more sensible to tie the archived objects in this way, rather
the somewhat vague setting of archive_timestamp to each object using
timezone_now().

zerver/migrations/0231_add_archive_transaction_model.py

@@ -0,0 +1,52 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.20 on 2019-06-23 21:20
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+import django.utils.timezone
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0230_rename_to_enable_stream_audible_notifications'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='ArchiveTransaction',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('timestamp', models.DateTimeField(db_index=True, default=django.utils.timezone.now)),
+                ('restored', models.BooleanField(db_index=True, default=False)),
+                ('type', models.PositiveSmallIntegerField(db_index=True)),
+                ('realm', models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, to='zerver.Realm')),
+            ],
+        ),
+        migrations.RemoveField(
+            model_name='archivedattachment',
+            name='archive_timestamp',
+        ),
+        migrations.RemoveField(
+            model_name='archivedmessage',
+            name='archive_timestamp',
+        ),
+        migrations.RemoveField(
+            model_name='archivedreaction',
+            name='archive_timestamp',
+        ),
+        migrations.RemoveField(
+            model_name='archivedsubmessage',
+            name='archive_timestamp',
+        ),
+        migrations.RemoveField(
+            model_name='archivedusermessage',
+            name='archive_timestamp',
+        ),
+        migrations.AddField(
+            model_name='archivedmessage',
+            name='archive_transaction',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, to='zerver.ArchiveTransaction'),
+        ),
+    ]