Bans and bulk deletes

[mniip]

Currently when a user gets banned, the gateway emits a bunch of bulk message deletes (one per channel, but could be multiple per channel if there's many messages), but could still emit single deletes if there's only one message in a channel. We'd like to log purged messages from banned members in the permanent log channel (not the temporary one). However telling apart which deletes correspond to a purge is not trivial.

One option is wait a little bit before reporting a delete, to see if any other deletes from the same user arrive, and if a "guild_member_ban" event happens in the meantime, we switch the reporting channel.

A simpler option is to log everything to the temporary channel, and when a "guild_member_ban" event happens, we additionally log all the messages we have for that user (we don't delete deleted messages from the db) in the permanent channel.