Smart staged rollouts

[kitallis]

Proposal

Tramline now supports tracking and driving staged rollouts (or phased releases). The next logical step is for it to be able to automatically do so without user intervention.

The scope here is clearly vast, there could be a variety of data sources we could track to make an informed decision. We could do anomaly analysis, we could do regression analysis from historical data and compute a score based on which we resume/halt rollouts.

What this achieves / problem(s) it solves

With #280 and this, we would have a system that is very close to a fully automatic pipeline. The MTTR would be much shorter, and computing data from various sources and then making an informed decision wouldn't be a human intervened activity anymore.

Additional context

We should pick some obvious simpler systems to integrate with first like Sentry and/or Crashalytics to see how effective we can be to our users.