This PR adds a destroy:posts rake task that can be used to hard-delete a list of posts. Useful for dealing with large amounts of spam that has been soft deleted and needs to go.
Notes:
Works on both non-deleted and soft-deleted posts. (We might want to change this to work on only soft-deleted posts?)
Works exclusively on post IDs. We can't mix topic and post IDs as they might clash, and we have no way of resolving that ambiguity.
Accepts either a rake-style array of IDs or, more conveniently, you can pipe the argument in through STDIN.
Added a confirmation step since it's a fairly destructive operation.
