export: Just export all streams in a realm.

Before this change we would only export streams
that had actual subscribers, which is usually
harmless, but it was mostly a relic of a one
time migration that we did when we were cleaning
up some dirty data in some of our very early
databases (circa 2016).

Now we work down the table hierarchy in a
more natural way:

    - get Streams in Realm
    - get Recipients matching above Streams
    - get Subscriptions matching above Recipients

Note that for per-user exports, I kept the
same logic (users -> subscriptions -> recipients ->
streams) we had before.

One subtle detail here is that we make our
final Config blocks--which build the final
version of Recipient/Subscription--now hang
off of realm_config.

Fixes #15146.

zerver/lib/export.py

@@ -741,29 +741,29 @@ def get_realm_config() -> Config:
     )
 
     #
-    stream_subscription_config = Config(
-        table='_stream_subscription',
-        model=Subscription,
-        normal_parent=user_profile_config,
-        filter_args={'recipient__type': Recipient.STREAM},
-        parent_key='user_profile__in',
+
+    stream_config = Config(
+        table='zerver_stream',
+        model=Stream,
+        exclude=['email_token'],
+        post_process_data=sanity_check_stream_data,
+        normal_parent=realm_config,
+        parent_key='realm_id__in',
     )
 
     stream_recipient_config = Config(
         table='_stream_recipient',
         model=Recipient,
-        virtual_parent=stream_subscription_config,
-        id_source=('_stream_subscription', 'recipient'),
+        normal_parent=stream_config,
+        parent_key='type_id__in',
+        filter_args={'type': Recipient.STREAM},
     )
 
     Config(
-        table='zerver_stream',
-        model=Stream,
-        virtual_parent=stream_recipient_config,
-        id_source=('_stream_recipient', 'type_id'),
-        source_filter=lambda r: r['type'] == Recipient.STREAM,
-        exclude=['email_token'],
-        post_process_data=sanity_check_stream_data,
+        table='_stream_subscription',
+        model=Subscription,
+        normal_parent=stream_recipient_config,
+        parent_key='recipient_id__in',
     )
 
     #
@@ -781,7 +781,7 @@ def get_realm_config() -> Config:
     # Now build permanent tables from our temp tables.
     Config(
         table='zerver_recipient',
-        virtual_parent=user_profile_config,
+        virtual_parent=realm_config,
         concat_and_destroy=[
             '_user_recipient',
             '_stream_recipient',
@@ -791,7 +791,7 @@ def get_realm_config() -> Config:
 
     Config(
         table='zerver_subscription',
-        virtual_parent=user_profile_config,
+        virtual_parent=realm_config,
         concat_and_destroy=[
             '_user_subscription',
             '_stream_subscription',