Simplify PhabricatorNotification and allow to use notifications:payload

Summary:
With rNOTIF67730e913d04, Phabricator began to match other services.

We continue to simplify Phabricator classes and standardize constructors.

This change allows:

• to use the notifications:payload command with Phabricator too
• to use the same rawContent format (inline JSON) than other notifications

Test Plan: php artisan notifications:payload Phabricator storage/logs/PhabricatorPastePayload.json Nasqueron

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T984

Differential Revision: https://devcentral.nasqueron.org/D635