Add mix task to send a notifaction, with a web push

Sending only a Push notification can't be done, it needs a Notification. Creating a notification needs an activity. Creating an activity needs an object. That's why it's running throughout the whole process.

It isn't really clean because it's forging a post, it might be acceptable to perform it from an account like "admin" or similar.

That said, you can't use Pleroma's builtin users ('relay' and 'internal.fetch') because they are invisible and stripped out from notification creation pipeline. It would have been a better solution, to have real administrative users.

Close #463