Implement sending email notifications via POST /notification

[mrkvon]

Implement endpoint that sends email notification to a person when called.

POST /notification with body

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "actor": {
    "type": "Person",
    "id": "https://sender.example.com/profile/card#me"
  },
  "object": {
    "type": "Note",
    "id": "https://example",
    "content": "message"
  },
  "target": {
    "type": "Person",
    "id": "https://receiver.example.com/profile/card#me"
  }
}