Add field to enable/disable comments at channel level

[tzarebczan]

It has been brought to our attention that many creators would rather disable comments on their content if we don't have proper moderation in place. This would be done via a channel level setting that would be respected by applications (can't actually prevent it on the comment server / blockchain solution).

I'd also argue we should do this on a per claim basis, but Jeremy said channels are a good place to start.