Stale Issues

[Rojikku]

I believe we should address the amount of stale issues by setting up the stale github action.
Might also be good for PRs that can't be merged. We can add exempt tags as appropriate.

Concept:

1. Issue is opened, person does not respond to comment.
2. After 14 days or so, issue is marked stale, sending a message saying so.
3. If there's no response after another 14 days or so, the issue can be automatically closed.

Github Stale Action