Add per-channel storage limits

[aaronpk]

Currently storage limits are set for a user's account and apply to everything they're following. However it would be great if limits could be configured per channel.

I have several channels which collect an obscene amount of data, and I really only dip in and out of them, and don't care about anything older than a few days. I'd like to be able to set those to delete everything older than 7 days, but still keep most of my channels archiving permanently.