Create get poll route

[ET33]

We need a /polls/:poll_id route that will respond data from the poll specified by poll_id.

It's expected to receive a GET request along with the user token (in the request header) and respond the poll data.

GET - /polls/:poll_id
Request:
    header: {
        "token": string
    }

Response:
    {
        "poll_id": int,
        "name": string,
        ...
    }