Automate uploading of stable ISOs

## Problem

Right now we can kick off a stable ISO build in the OS repo, and it gets uploaded to DO. However, the process of getting that ISO up on the CDN and available to users is manual, and only @lewisgoddard can currently do it. This creates a human bus factor that may become more apparent as we resume our monthly cadence of ISO refreshes.

## Proposal

@btkostner and I talked briefly (and he has more thoughts!) about how that last step could be automated. He mentioned something like an RSS feed from the builds site to seed @lewisgoddard's CDN boxes. I'm sure he can share more. :)

## Prior Art