Add a little check to make sure the pre-commit hooks ran as expected

I've made this into a comment rather than a failing check, because
I've noticed instances where the pre-commit hook doesn't do the right
thing.

For example, djhtml refuses to process templates/navigation.html,
because it contains a long line. This is because the crown logo SVG from
the header contains a very long path attribute.

.github/workflows/check-pre-commit-has-been-run.yml

@@ -0,0 +1,28 @@
+name: Check pre-commit hooks ran
+
+on:
+  - pull_request
+  - push
+
+jobs:
+  check-precommit-hook:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check that the pre-commit hook has been run
+        id: check
+        run: |
+          set +e
+          output=$(pre-commit run --from-ref origin/HEAD --to-ref HEAD 2>&1)
+          exit_code=$?
+
+          if [[ $exit_code -eq 0 ]]; then
+              echo "Pre-commit hook ran successfully"
+          else
+              PR_NUMBER=${{ github.event.pull_request.number }}
+              GITHUB_TOKEN=${{ secrets.GITHUB_TOKEN }}
+              COMMENT="⚠️ Make sure you have [pre-commit](https://pre-commit.com]) installed.\n\npre-commit output: ${output}"
+
+              echo $output
+              gh pr comment $COMMENT_URL "https://api.github.com/repos/${{ github.repository }}/issues/${PR_NUMBER}" -b $COMMENT
+          fi