Replace JS confirm() dialog with "undo" option for deleting posts

When a user attempts to delete a post, instead of presenting him with a Javascript `confirm()` dialog (which blocks the UI and is frustrating as hell), go ahead and delete the post (or simply mark it as deleted so it doesn't display in the list of visible posts)

Then display a link in a Bootstrap `.success` alert msg that gives him the option to "undo" the post removal.
