Add optional time parameter to playlist duplication

[RocketMan]

This PR adds an optional fromtime parameter to the playlist duplication/rebroadcast API. The value may be a range (hhmm-hhmm) or a time (hhmm). If you specify a time, the API copies from the specified time to the end of the playlist.

Example:

Playlist 12345 runs from 0000-0300. To duplicate only one hour of this playlist 0100-0200 for rebroadcast on 2022-01-01 at 1800-1900:

POST /api/v1/playlist HTTP/1.1
X-APIKEY: eb5e0e0b42a84531af5f257ed61505050494788d
Content-Type: application/vnd.api+json

{
  "data": {
    "type": "show",
    "attributes": {
      "rebroadcast": true,
      "date": "2022-01-01",
      "time": "1800-1900"
    },
    "meta": {
      "fromtime": "0100-0200"
    },
    "relationships": {
      "origin": {
        "data": {
          "type": "show",
          "id": "12345"
        }
      }
    }
  }
}

More details are available in docs/Playlists.md