update zulip from RChain mirror incrementally

[dckc]

@steverosstalbot suggested we need a means of enabling other zulip instances to receive the same notifications (selectively) as RChain to enable them to stay up to date.

I'm not sure how to make sense of this without the ability of those other zulip instances to write to RChain; i.e. this seems to imply full N-way-sync. I suppose that's coherent if we integrate at the django ORM level (#23) and skip the per-instance postgres service.

[dckc]

Last Saturday, @steverosstalbot shared some slides:

• DN-RCHAIN-SRT-001 Lifecycle of zulip postgres data on
  RChain