Delete erroneously uploaded package versions

This is motivated by the fact that last week I mistakenly uploaded an weird stack version ([9.9.9](http://hackage.haskell.org/package/stack-9.9.9)). I asked the Hackage admins to remove it, but was told that there is currently no ability to delete package versions in hackage. Admins did tell me that it's not a difficult feature to add and that it's something they want to do, so posting this issue to make it easier to track it.

Aside from fixing dumb mistakes, there could be important reasons this needs to be done, such as copyright violation or other illegal content.

Deprecating a version helps a bit, but I found it did not prevent `cabal install stack` from installing the 9.9.9 version (however, giving it impossible dependency version bounds did the trick).

I am happy to work on a PR to implement this myself, but it would be most helpful to have someone nudge me in the right direction before I start.