Follow a user

{
  "id": "string",
  "type": "string",
  "initiator": "string",
  "target": "string",
  "created_at": 0,
  "updated_at": 0
}

{
  "error": {
    "message": "You are not logged in.",
    "code": 7,
    "status": 401
  },
  "data": null
}

{
  "error": {
    "message": "User not found.",
    "code": 2,
    "status": 404
  },
  "data": null
}

{
  "error": {
    "message": "{0} already exists with {1}.",
    "code": 6,
    "status": 409
  },
  "data": null
}