report full standard validation results for standard candidate chains to new slack channel

[geoknee]

These are useful to get from time to time, for an update on the question “what blocks this chain from becoming standard”?

Implementation sketch:

• get a list of chains which are standard_candidates (inspect chain.StandardChainCandidate)
• just promote-to-standard chainID, or you could duplicate the Go code which does that
• invoke the full set of standard validation checks
• report to new slack channel on failure (but we need a way to separate / indicate this is a special failure, maybe a new slack channel would be fine)