Support disabling issues and pulls on a per-repo basis #444

I have a few repos that I would love to share the code/content for, but do not want anyone filing issues/opening pull requests (i.e. a repo for a personal website). Being able to declare that those venues of contribution are not open in the interface would be fairly useful.

This isn't a problem yet, but both issues and pull requests have been a source of spam on other git forges, and this would help preempt that issue.