Clean up invalid image links in old threads

[E9A7F3bD]

I've seen most new threads are using "attachments" and these images are hosted by FLM, so they're fine as long as FLM is online.

But a lot of old threads are filled with things like these:

Spoiler: [Image] [Image]

They looked annoying and your browser is just constantly sending invalid requests and pings to these sites.

I think it's necessary for us to do some cleaning, detect all these known external image hosting links, batch editing and deletion which are invalid.

Where possible, maybe back up those external images that are still valid so that we could replace and restore them in the future.

Almost most external image hosters are "ephemeral" and they end up falling down for various reasons.

I believe everyone can realize the importance of these movie previews to threads, and they are an important way for us to perceive content.