Investigate usage of bot for marking issues as stale

[alamers]

There are bots (e.g. https://github.com/apps/stale) that can help us manage open tickets. As discussed there are quite some tickets that seem to be stale (no activity for a long period). Automatically marking them as 'stale' would help us to discuss these whether to keep them open or close them.

[ahokkonen]

sound good and setup seems to be pretty simple. I'm voting for taking it into the action :)

[alamers]

Is resolved in #249 and seems to work ;)

We should be careful since this is the first run that some issues may be closed relatively quickly without being discussed. But I guess we'll get a mail for those as well so we can reopen them.