Add: [GitHub] enable probot/stale, to flush out stale issues/pull requests

Stale bot checks if an issue hasn't had any attention in the last
60 days, or if a pull request hasn't had any attention in the last
30 days. If so, it marks it as stale. After 7 more days, it closes
the issue/pull request. Any activity on the issue/pull request
resets the timer.

This helps keeping the counter low, and the work manageable. There
is a lot of reasoning behind this idea, and other projects show it
really helps to keep the issue/pull request count low.

More information here: https://github.com/probot/stale