Add pull request preview actions

.github/workflows/preview_cleanup.yml

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+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

.github/workflows/preview_pull_request.yml

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+name: Preview Pull Request
+
+on:
+    pull_request:
+        types: [synchronize, reopened, labeled]
+
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        # make sure the pull request is labeled with 'use-preview'
+        if: github.event.label.name == 'use-preview' || contains(github.event.pull_request.labels.*.name, 'use-preview')
+
+        steps:
+            - uses: actions/checkout@v2
+              with:
+                  # Head commit of the pull request
+                  ref: ${{ github.event.pull_request.head.sha }}
+                  submodules: recursive
+
+            - name: Setup Node
+              uses: actions/setup-node@v2
+              with:
+                  node-version: 15
+                  cache: "yarn"
+
+            - run: yarn install
+            - run: yarn check
+
+            - name: build
+              run: yarn build
+
+            - name: publish preview
+              uses: JamesIves/github-pages-deploy-action@4.1.5
+              with:
+                  branch: build-previews
+                  folder: dist
+                  target-folder: ${{ github.ref }}
+
+            - name: send comment
+              uses: marocchino/sticky-pull-request-comment@v2
+              with:
+                  header: Preview environment
+                  message: |
+                      ## Preview environment
+                      https://${{ github.repository_owner }}.github.io/revite/${{ github.ref }}/index.html
+
+                      This link will remain active until the `use-preview` label is removed.