Add GitHub Action that immediately closes issue / PRs

Unfortunately, it's not possible to disable Pull Requests on the
mirrored repository on the GitHub, so this commit adds external action
that closes any new open Issue or Pull Requests instead letting them rot
unnoticed.

.github/workflows/lockdown.yml

@@ -0,0 +1,15 @@
+- name: 'Lock down mirror repository'
+
+on:
+  issues:
+    types: opened
+  pull_request:
+    types: opened
+
+jobs:
+  lockdown:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/repo-lockdown@v2
+        with:
+          github-token: ${{ github.token }}