Add support to delete a challenge

[ThomasKranitsas]

Since we now support challenges with only { status: 'New', name, projectId }, it would be a good idea to have a way to delete a challenge so we don't end up with a ton of half-baked challenges.

This could be:

• a DELETE /challenges/:uuid endpoint
• a processor that will automatically expire New challenges after last modified > 30 days
• both?