[chat_history] add ability to require at least n items pinned

[kevmo314]

This might address Eric's feedback if we add an option that mandates that some number of items are pinned, regardless of their pin duration. The logic should be:

If there are fewer than n items that should be pinned, choose the last n items by pin expiration and pin them.

[SputNikPlop]

Just an update on this but an api endpoint for this has been updated to planned per https://twitch.uservoice.com/forums/310213-developers/suggestions/45606919-pinned-messages-api