Add calibreapp/image-actions (#499)

This should optimize any new images for us in PRs.

Signed-off-by: XhmikosR <xhmikosr@gmail.com>

.github/workflows/calibreapp-image-actions.yml

@@ -0,0 +1,24 @@
+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@1.1.0 # TODO: if they start using a tag like v1, switch to that
+        with:
+          githubToken: ${{ secrets.GITHUB_TOKEN }}