GitHub: automatically close inactive issues

If an issue has received no input after a configurable interval (e.g. 90 days), then close it as inactive.
Readme:

Out of the box the following things constitute activity:

  • opening the issue
  • commenting on the issue
  • adding or removing a label from the issue
  • assigning/unassigning the issue
  • editing the issue

When the issue is closed, a message will be posted stating why.

⚠️ Note: out of the box this will only operate on issues that see some activity from the time you deploy it onwards (i.e. it won't clean up old issues that no-one touches); however if you enter developer mode you can double click the manual event at the bottom of the sketch to automatically go through up to 100 existing open issues and add them to the database for monitoring (if you need more than 100 issues, you can customise the sketch to paginate through the results; or tell me you need that and I'll do it: @Benjie)