Introduce a Stale bot for abandoned issues.

[MisterTimn]

There are some issues which have been open for a while and are either abandoned, already solved (but not closed by issuer) or outdated.

We can setup a Stale bot to flag these issues as stale and either automatically close them or allow us to filter Stale issues and manually process them. Relevant issues can be omitted due to their labels and for the rest we can rely on the stale bot.

[RubenVerborgh]

I do support that. Maybe there could be an exception for certain labels, that we specifically assign.
Feel free to take this on, @MisterTimn!

[joachimvh]

I'm not sure yet how I think about this as I'm afraid it might increase the workload (as there would be more notifications to respond to), but can't be sure until I see it in action.

[MisterTimn]

Yes this might add more notifications but if we let it ignore labels it would only cause notifications for issues that are 'unmanaged' or dangling. A benefit also being that the issuer itself is notified, they can remove the stale label if they deem the issue is still relevant/unanswered and leave it be if not.

We will however have to find a good config that works for CSS.