Add a GitHub workflow cache to manually purge the Cloudflare cache

We had problems with the cache-purging logic in
#2086, and then also in
#2093. The symptom is that the
CSS isn't loading, because the front-page still serves an older version
(one that references a no-longer-existing CSS). The culprit seems to be
the un-purged Cloudflare cache.

Let's add a way to manually purge the cache. It is totally possible that
this is one of those famous "Close Door" elevator buttons that fool us:
https://www.sciencealert.com/the-close-door-buttons-in-elevators-don-t-actually-do-anything
But in case it isn't, let's have a way to trigger a purge.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/purge-cache.yml

@@ -0,0 +1,27 @@
+name: Purge Cloudflare Cache
+
+on:
+  workflow_dispatch:
+
+jobs:
+  purge-cache:
+    concurrency:
+      group: "purge-cache"
+      cancel-in-progress: false
+    runs-on: ubuntu-latest
+    steps:
+      - name: Purge Cloudflare cache
+        shell: bash
+        env:
+          CLOUDFLARE_TOKEN: ${{ secrets.CLOUDFLARE_TOKEN }}
+          CLOUDFLARE_ZONE: ${{ secrets.CLOUDFLARE_ZONE }}
+          ${{ inputs.cloudflare-zone }}
+          ${{ inputs.cloudflare-token }}
+        if: env.CLOUDFLARE_TOKEN != ''
+        run: |
+          curl "https://api.cloudflare.com/client/v4/zones/$CLOUDFLARE_ZONE/purge_cache" \
+            -H "Authorization: Bearer $CLOUDFLARE_TOKEN" \
+            -H "Content-Type: application/json" \
+            -d '{ "purge_everything": true }'
+
+