support per-channel user IDs

[nminchow]

Per channel IDs would be neat.

I think the basic implementation would be an additional table mapping discord IDs to a channel seed. (similar to the current per-server seed.) When a message is submitted to a channel, this table would be checked to see if the target channel has channel specific IDs enabled.

This will further complicated the (already complex) ban logic, especially in the context of rotating IDs. Essentially - the ban command and all logic that checks for bans would need to be channel aware.

requested by @kytebabe in the discord