Improve unread behavior

[12944qwerty]

Improves the color on the channel items when unread and resets unread when you send a message.

[cryptoAlgorithm]

Thanks for the contribution. I'd think that in addition to updating the unread state in the cache, calling the "ack" endpoint to update it with Discord would also be a good idea. I'd have to add the endpoint into DiscordKit before this can be implemented though, so I'll be keeping this open for now.