governance: automatically close stale PRs (stalebot / probot-stale)

I propose that we use a Github bot to automatically close PR/issues after a period of inactivity.

I've seen lot of big projets using that kind of bot (such as Travis, Docker, ...): I was mixed about it at the beginning, but since @monperrus enforce the closing of PR/issues manually to maintain a "clean" repository, I really think it might be useful to avoid doing ourselves this boring task.

An example of such as bot is available there: https://probot.github.io/apps/stale/