ci: automatically compress all images on PRs (#914)

.github/workflows/images.yml

+name: Compress Images
+on:
+  pull_request:
+    paths:
+      - '**.jpg'
+      - '**.jpeg'
+      - '**.png'
+      - '**.webp'
+jobs:
+  build:
+    # Only run on Pull Requests within the same repository, and not from forks.
+    if: github.event.pull_request.head.repo.full_name == github.repository
+    name: calibreapp/image-actions
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repo
+        uses: actions/checkout@v2
+      - name: Compress Images
+        uses: calibreapp/image-actions@main
+        with:
+          githubToken: ${{ secrets.GITHUB_TOKEN }}
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