Add GitHub Stale bot for issue management

[lampwins]

Proposed Changes

The core maintainers have discussed and decided to try out the GitHub Stale bot to assist in managing the flow of incoming issues. Stale bot will be added to the org and enabled on the netbox repo. To start with, issues will be marked as stale after 14 days of no activity. Then after 7 more days of inactivity, the issue will be closed. Any issue with either the "status: accepted" or "status: gathering feeback" labels applied will be exempt from all Stale bot actions.

This policy will be published in our CONTRIBUTING guide and is subject to change over time.

Justification

The flow of incoming issues to netbox has greatly increased in recent months. Given this added strain on the maintainers, it is natural that some new issues get more attention than others. Often this is a metric of an issues's overall usefulness to the project. In other cases in which issues mearly get lost in the shuffle, notifications from Stale bot can bring renewed attention to potentially meaningful issues.

Stale bot will be used as a tool to help the maintainers keep up with and groom the ever-growing backlog of work.

[Grokzen]

@lampwins Three questions. Is 21 days short for issues to be atomatically closed? Some of the issues on the main-page would be closed with these esttings. Two, will you maintainers do a pass on all current open issues to check them for the 2 labels that will stop it from being stale? I have found plenty of them from the past that is either blocked or have only UI as label. Three, consider adding Blocked as a option to be excluded from the bot? There is some issues on older pages that had this case and they would then be stale with these settings.

[jeremystretch]

Is 21 days short for issues to be atomatically closed?

Honestly, no. If no one responds to the issue in three weeks either no one feels it needs to be addressed or we're all dead.

Two, will you maintainers do a pass on all current open issues to check them for the 2 labels that will stop it from being stale?

Yes; I just completed a good amount of cleanup prior to merging @lampwins' PR.

Three, consider adding Blocked as a option to be excluded from the bot?

Done.

There is likely to be some trial and error involved as we learn what workflow makes the most sense with regard to stalebot. Rest assured that the maintainers team will adapt the policy as necessary.