Add optional setting for server wishlists

[benjamin-musil]

If enabled, a server can add/remove games to a wishlist with a price point or price cut requirement and get a notification from the bot when a game satisfies that requirement.

Useful for smaller servers, could also be useful for a server for a particular game and they want to notify users when their game is on sale.

On that note, another optional setting could be for only a server admin to add/remove games from a wishlist.

[acdvs]

There isn't a webhook feature to get price updates, so this would require constant polling of potentially thousands of games.

[benjamin-musil]

Limit number of wishlist titles to 5-10 total, likely there'll be repeats between servers. Deals usually last several hours at minimum unless there's a crazy flash sale, so infrequent polling wouldn't be an issue to me.

Worst case, framework is there for it if a webhook is introduced later or the bot infrastructure upgrades to handle a lot more traffic