Ability to preview commits to be deployed, and email authors that their stuff is going out

[untitaker]

Given an incident in sentry I think it would be nice if hitting deploy would send an email to all commit's authors before executing, to give them a little bit of time to sprint to the computer and watch dashboards.

We have something like this in Sentry releases, but the way we set it up currently only emails after the deploy has gone out.

Parts of this (the emailing part) might make sense to actually implement in Sentry releases, not Freight.