Issue 3494: Moderators can delete PromptMeme sign-ups

Resolves issue: http://code.google.com/p/otwarchive/issues/detail?id=3494

Added a 'Delete Sign-up' button to each of the Sign-ups listed under /requests for PromptMeme challenges. Moderators have access to this button without having to 'Edit' a sign-up, which had the potential to break semi-anonymous prompts.