Add GitHub Action to tidy stale issues

.github/workflows/stale.yml

@@ -0,0 +1,17 @@
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: "0 0 * * *"
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/stale@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-issue-message: 'This issue has been inactive for 60 days. If the issue is still relevant, please comment to re-activate the issue. If no action is taken within 7 days, the issue will be marked closed.'
+        stale-pr-message: 'This pull request has been inactive for 60 days. If the pull request is still relevant, please comment to re-activate the pull request. If no action is taken within 7 days, the pull request will be marked closed.'