workflow: add stale action (#2196)

Setup to mark stale after 300 days, and close 30 days after marked stale
unless the issue is commented on, or has the `stale` label removed.

The 'never stale' label can be used to exempt an issue from ever being
considered stale.

Fixes: #2190

.github/workflows/stale.yml

@@ -0,0 +1,17 @@
+name: "Close stale issues"
+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 is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days'
+        stale-issue-label: 'stale'
+        exempt-issue-label: 'never stale'
+        days-before-stale: 300
+        days-before-close: 30