Automatically flush Cloudflare caches

[dfabulich]

The button exists now. I didn't wind up automating the process because it's a nuisance for zip files:

• Replace the file
• Clear the cache for the zip file
• Rebuild Master-Index
• Wait five minutes for Unbox to notice the new Master-Index
• Recheck Unbox

If you check Unbox too soon, it may still have the old zip; then you'd have to clear the cache again.

Originally posted by @erkyrath in #34 (comment)

I think we can do better than this. I filed a PR iftechfoundation/ifarchive-unbox#48 for Unbox to allow forcing it to refresh its cache of the master index. If we were to merge that PR, we could have "rebuild master-index" automatically refresh unbox's cache, and then replacing the zip file could do all of those steps automatically.