[dmd-internals] Auto-warn authors followed by auto-close of very stale PRs

[Steven Schveighoffer via dmd-internals]

> On May 9, 2016, at 9:01 AM, Leandro Lucarella via dmd-internals <dmd-internals at puremagic.com> wrote:
> 
> BTW, having old broken PRs consumes resources in the auto-tester, right?
> Once a PR have rotten it will be never magically fixed, so it doesn't
> make sense to keep testing it with the auto-tester.

This consumes nearly zero cycles on the auto tester (if it ever gets there, newer PRs are tested first), because each test fails instantly with not being able to merge.

I think we should leave them open. Note that I think a non-member cannot reopen a closed PR that was closed by a member. At least someone told me once that the PR I closed of theirs they could not reopen.

-Steve