Support handling and feedback for Discord slowmode

[Half-Shot]

This is blocked on https://github.com/discordjs/discord.js/issues/2946

[Half-Shot]

It's actually done and in master https://discord.js.org/#/docs/main/master/class/TextChannel?scrollTo=rateLimitPerUser , we just need it to get released.

[Half-Shot]

The way we should handle this:

• Hook into something that can let us listen for rate limit changes.
• If the channel gets rate limited, we should send a message into the room to let the users know the limits.
• We can't stop matrix users from talking in a rate limited channel, but we should refuse to relay messages that are outside of the interval rule by applying our own rate limiting inside the bridge.
• This would be quite disruptive for users, so we would have to do some sort of m.relates_to thing to show in the UI that the message was not bridged.