have the option to download pgns of a season/round

[glbert-does]

it would seem quite reasonable to me to offer this.
we could use the lichess api: https://lichess.org/api#tag/Games/operation/gamesExportIds
to collect them by game id. however, lichess currently limits this to 300 game ids, in some team seasons this might not even cover one round, so we would have to make multiple requests.

if we implemented #531, then lichess would basically cover this for us already via https://lichess.org/api#tag/Broadcasts/operation/broadcastAllRoundsPgn

so possibly the easiest (or at least most useful) way to get this would be to implement #531 first.