Code review comment for ubuntu-archive-tools:frozen-for-release

I think this would be fine in principle. A couple of things I'm unsure about:

1) It fails unsafe. If for example Launchpad has an issue that returns 500, then release will be permitted. Should we instead require positive confirmation that the series is not in freeze? Not being able to release if git.launchpad.net is unavailable doesn't seem too bad to me, especially because --override-freeze exists.

2) Is the britney hints directory the right place to do this? For now, doesn't britney consider all stable releases as inactive? It seems a bit strange to overload this, but I don't feel strongly about this.

But also, if this works, it's better than the current situation, so let's land it and fixes and improvements can come later.