Automatically close invalid PRs using GitHub Actions (mmistakes#3313)

* Try auto-closing bad PRs

* Include empty PR body as well

* Add "Type: Invalid" label as well

.github/PULL_REQUEST_TEMPLATE.md

@@ -23,4 +23,9 @@
 
 <!--
   Is this related to any GitHub issue(s)?
--->
+-->
+
+<!--
+  Please delete this comment if you confirm that you want to submit this Pull Request.
+  CHECK_PR_DID_NOT_CONFIRM
+-->

.github/workflows/bad-pr.yml

@@ -0,0 +1,18 @@
+name: Cleanup bad PR
+
+on:
+  pull_request_target:
+    types: [opened, reopened]
+
+jobs:
+  close-pr:
+    runs-on: ubuntu-latest
+    if: "contains(github.event.pull_request.body, 'CHECK_PR_DID_NOT_CONFIRM') || github.event.pull_request.body == ''"
+    steps:
+      - uses: actions-ecosystem/action-add-labels@v1
+        with:
+          labels: 'Type: Invalid'
+      - uses: superbrothers/close-pull-request@v3
+        with:
+          # Optional. Post an issue comment just before closing a pull request.
+          comment: "This PR is not valid for inclusion. Please check again if you're submitting improvements for *the theme*."