This commit renames the 'PRIVATE_MESSAGE' attribute of the
'NotificationTriggers' class to 'DIRECT_MESSAGE'.
Custom migration to update the existing value in the database.
It includes 'TODO/compatibility' code to support the old
notification trigger value 'private_message' in the
push notification queue during the Zulip server upgrades.
Earlier 'private_message' was one of the possible values for the
'trigger' property of the '[`POST /zulip-outgoing-webhook`]' response;
Update the docs to reflect the change in the above-mentioned trigger
value.
Prakhar Pratyush