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/lib/export.py

@@ -88,6 +88,7 @@ ALL_ZULIP_TABLES = {
     'zerver_attachment_messages',
     'zerver_archivedreaction',
     'zerver_archivedsubmessage',
+    'zerver_archivetransaction',
     'zerver_botconfigdata',
     'zerver_botstoragedata',
     'zerver_client',
@@ -176,6 +177,7 @@ NON_EXPORTED_TABLES = {
     'zerver_archivedattachment_messages',
     'zerver_archivedreaction',
     'zerver_archivedsubmessage',
+    'zerver_archivetransaction',
 
     # Social auth tables are not needed post-export, since we don't
     # use any of this state outside of a direct authentication flow.