Make per-user permissions override per-role permissions and allow owners to constrain their own permissions

Make per-user permissions override per-role permissions instead of the current situation where per-role and @everyone permission overrides per-user ones in channel management so that per-user permissions and revocations always overrides role-granted permissions. Also have a per-user toggle to disable slow-mode bypass of moderators and server owners (yeah, in some cases, you need rate limits not even owner is supposed to bypass).