Add route for admins to create user sessions

[gr2m]

Follow up for #14

As an admin, I would like to be able to create a user session, so I can debug an account without messing with passwords.

I would suggest the route POST /account/{id}/sessions for that. Without any body (does JSON API allow that?).

The response would look like this:

{
  "links": {
    "self": "https://example.com/accounts/abc1234/sessions/sessionid123"
  },
  "data": {
    "id": "sessionid123",
    "type": "session",
    "relationships": {
      "account": {
        "links": {
          "related": "https://example.com/accounts/abc1234"
        },
        "data": {
          "id": "abc1234",
          "type": "account"
        }
      }
    }
  }
}

This implies that we would also need a GET /accounts/{id}/sessions/{id} and a DELETE /accounts/{id}/sessions/{id} route, which works for me.

Any objections / thoughts?