Add pull request preview actions

.github/workflows/preview_cleanup.yml

@@ -0,0 +1,36 @@
+name: Clean Preview
+
+#! Safety:
+#! this workflow should not execute any code at all
+#! see githubs docs on pull_request_target for more
+on:
+    pull_request_target:
+        types: [unlabeled]
+
+jobs:
+    clean:
+        runs-on: ubuntu-latest
+        if: github.event.label.name == 'use-preview'
+
+        steps:
+            - uses: actions/checkout@v2
+              with:
+                  ref: build-previews
+                  persist-credentials: false
+
+            - name: clean previews
+              run: 'rm -rf "./refs/pull/${{ github.event.pull_request.number }}"'
+
+            - name: publish cleaned previews
+              uses: JamesIves/github-pages-deploy-action@4.1.5
+              with:
+                  folder: .
+                  branch: build-previews
+
+            - name: send comment
+              uses: marocchino/sticky-pull-request-comment@v2
+              with:
+                  header: Preview environment
+                  message: |
+                      ## Preview environment
+                      There is no longer a preview enviroment for this pull request due to the `use-preview` label being removed