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.
This commit replaces the `create_stream_by_admins_only` setting with a
new `create_stream_policy` setting, which mirroring the structure of
the existing `invite_to_stream_policy`.
This is important preparation for migrating the waiting period feature
to be its own independent setting.
Fixes#12236.
Tim Abbott
Harshit Bansal
David Wood