Add message pruning command for data autonomy

[Kardbord]

A request from a discord user:

I have an odd request for some kind of bot function that can bypass the 14 day limit on discord's mass message deletion command for bots.

Ideally it'd be something like "/prune #Channel" and then would, in a loop, find and individually delete every message I may have hypothetically sent to this channel over the last 30 years. Would that be simple? It'd have to have a sleep function in there or something to avoid getting rate limited, I'm sure.

The whole goal here is to give people in a server full data autonomy over what they've posted

For reference, the referenced 14 day limit seems to have been added in discord/discord-api-docs#208.

This is a feature I really like the idea of, but could be tricky to navigate. I don't want to get flagged for API abuse. I'll have to do some thinking on it.