Add per-user cooldown for opening modmails

[anden3]

We've had a few occurrences of malicious users spamming modmails to annoy staff, and poise has a way to do cooldowns, so it might just be a matter of adding user_cooldown = 60 to the #[poise::command] for modmail.

https://docs.rs/poise/latest/poise/macros/attr.command.html#cooldown

[anden3]

Seems like we need to store a cooldown tracker in user data and query it inside https://github.com/rust-community-discord/ferrisbot-for-discord/blob/main/src/commands/modmail.rs#L91.