GitHub stale action

Add a GitHub action to mark issues and PRs as stale and
to eventually close them after a grace period.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

.github/workflows/stale.yml

@@ -0,0 +1,25 @@
+name: Mark stale issues and pull requests
+
+# Please refer to https://github.com/actions/stale/blob/master/action.yml
+# to see all config knobs of the stale action.
+
+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 had no activity for 30 days. In the absence of activity or the "do-not-close" label, the issue will be automatically closed within 7 days.'
+        stale-pr-message: 'This pull request had no activity for 30 days. In the absence of activity or the "do-not-close" label, the pull request will be automatically closed within 7 days.'
+        stale-issue-label: 'stale-issue'
+        stale-pr-label: 'stale-pr'
+        days-before-stale: 30
+        days-before-close: 7
+        exempt-pr-label: 'do-not-close'