Allow projects to be deleted

[jasonaowen]

Allow the author of a project to delete it.

We should probably leave behind a special web page that indicates the project has been deleted; we can probably do that by adding an exception that maps to HTTP 410 and an error handler page for HTTP 410. That'd also require recording a list of deleted projects; for privacy's sake, though, I don't think we want to record who deleted it or what the project was before it was deleted.

[jasonaowen]

In multi-author projects (#37), I think an author should not be able to delete the project, but can only remove themself as an author. Once all but one of the authors have removed themselves, the project becomes a single-author project and can be deleted by the remaining author.