Add `Orchestra\Model\User::notify()` method which utilize `Orchestra\Notifier\NotifiableTrait`

Signed-off-by: crynobone <crynobone@gmail.com>
Fixes use statement.

Signed-off-by: crynobone <crynobone@gmail.com>
Add 2.2 changelog.

Signed-off-by: crynobone <crynobone@gmail.com>