Adds a "Mobile push notifications enabled by default" checkbox to the
channel settings General tab (between channel folder and email address)
for existing channels. In the channel creation form, the checkbox
appears at the end of the Subscription management section and is
only shown to organization administrators.
When subscribing to a stream with default_push_notifications=True,
the new subscription gets push_notifications=True. Live stream
update events keep the setting in sync without a reload.
Document the setting with a new help center article, and cross-link it
in relevant help center articles.
Fixes#23873.