Pick issues more randomly

[yrodiere]

Currently, we notify about issues from the most recently updated to the least recently updated, with some rules to skip some issues (notified recently, ...) that help avoid repeated notifications.

For smaller pools of issues, this is sufficient, as we will eventually end up in a situation where all issues have been notified recently.

For larger pools of issues, however, this might be a problem, as the first notified issues are likely to be notified again before we went through the whole pool. So some of the least recently updated issues will never get a chance to be notified.

It would be nice to introduce some randomness, to sometime notify about older issues, in paticular when it comes to the "Maintenance/Stale" category or "Stewardship" (#36).

Implementation-wise, that might be tough. Indeed, GitHub does not (to the best of my knowledge) offer a way to pick an issue at random, so all we can do is go through issues sorted in some order. If we want to give every issue an equal probability of being picked, we might have to go through the whole list, which means going through thousands of issues... which is unlikely to scale. So, that will require some thought.