Check boxes which refererence closed issues

[mdinger]

rust-lang/rust#20761 is populated with checkboxes like below:

• Is this bug closed Check boxes which refererence closed issues #44:

Highfive could check the boxes anytime the issues referenced are closed.

[mdinger]

Bors could too. I can move this to bors if you think that's a better place for it.

[nrc]

Probably better for bors to do it, since that bot is already responsible for closing issues

[mdinger]

Moved to barosl/homu#52