subscriptions: Migrate notification setting defaults model.

This commit migrates the Subscription's notification fields from a
BooleanField to a NullBooleanField where a value of None means to
inherit the value from user's profile.

Also includes a migrations to set the corresponding settings to None
if they match the user profile's values. This migration helps us in
getting rid of the weird "Apply to all" widget that we offered on
subscription settings page.

The mobile apps can't handle None appearing as the stream-level
notification settings, so for backwards-compatibility we arrange to
only send True/False to the mobile apps by applying those defaults
server-side.  We introduce a notification_settings_null value within a
client_capabilities structure that newer versions of the mobile apps
can use to request the new model.

This mobile compatibility code is pretty effectively tested by the
existing test_events tests for the subscriptions subsystem.

zerver/lib/stream_subscription.py

@@ -1,4 +1,4 @@
-from typing import Dict, List, Tuple
+from typing import Any, Dict, List, Tuple
 from mypy_extensions import TypedDict
 
 from django.db.models.query import QuerySet
@@ -74,3 +74,30 @@ def num_subscribers_for_stream_id(stream_id: int) -> int:
     return get_active_subscriptions_for_stream_id(stream_id).filter(
         user_profile__is_active=True,
     ).count()
+
+
+def handle_stream_notifications_compatibility(user_profile: UserProfile,
+                                              stream_dict: Dict[str, Any],
+                                              notification_settings_null: bool) -> None:
+    # Old versions of the mobile apps don't support `None` as a
+    # value for the stream-level notifications properties, so we
+    # have to handle the normally frontend-side defaults for these
+    # settings here for those older clients.
+    #
+    # Note that this situation results in these older mobile apps
+    # having a subtle bug where changes to the user-level stream
+    # notification defaults will not properly propagate to the
+    # mobile app "stream notification settings" UI until the app
+    # re-registers.  This is an acceptable level of
+    # backwards-compatibility problem in our view.
+    assert not notification_settings_null
+
+    for notification_type in ["desktop_notifications", "audible_notifications",
+                              "push_notifications", "email_notifications"]:
+        # Values of true/false are supported by older clients.
+        if stream_dict[notification_type] is not None:
+            continue
+        target_attr = "enable_stream_" + notification_type
+        if notification_type == 'audible_notifications':
+            target_attr = "enable_stream_sounds"
+        stream_dict[notification_type] = getattr(user_profile, target_attr)