Delete versions / crates

[glueball]

Crates.io only supports yanking, so other projects/crates, that are locked to a specific version do not break. However, in a private environment, like ktra, it is safer to remove old versions/crates since you can control whether other crates are locked to that particular version.

This should be implemented as an extra web API, since cargo only has the yank subcommand. For extra safety, the delete API should only work on versions that are already yanked, and that are not depended on by any (non-yanked) crate in the same repository.