want a way to lock/embargo/privatize releases and downloads

It would sometimes be useful to make a release and its download files not publicly accessible, somewhat similarly to the embargo feature on announcements. A couple of cases:

1- We want to announce and generally recommend the release when binaries for all platforms have been built. Various people build the binaries and upload them over several days. It may cause user confusion if the current release doesn't have a full set of binaries yet.

2- If we discover a release is seriously buggy, we might want to disable access to it. At the moment the only option is to delete all the files. It would be nice to block access (possibly with a 'yes, really' link), showing a project-provided message.

In either case:

- project team members should still have access
- there should be a prominent decoration (as for private objects) that the release is not visible

Possibly this is just that releases should have a privacy bit, though the case of withdrawn releases seems different.