POST /v2/reporting-entities/:id/portfolios

Create a new portfolio under a reporting entity. The accounts and
depositories in the portfolio are linked here.

Path params:
  id (required) — Reporting-entity identifier.

Request:
{
  "data": {
    "type": "portfolio",
    "attributes": {
      "name": "string (required)"
    },
    "relationships": {
      "accounts":     { "data": [ { "type": "account", "id": "string" } ] },
      "depositories": { "data": [ { "type": "depository", "id": "string" } ] }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "portfolio",
    "attributes": {
      "name": "string"
    },
    "relationships": {
      "reportingEntity": { "data": { "type": "reporting-entity", "id": "string" } },
      "accounts":        { "data": [ { "type": "account", "id": "string" } ] },
      "depositories":    { "data": [ { "type": "depository", "id": "string" } ] }
    }
  }
}