Spike: Investigate Thumbnail Poisoning

[jgrim]

Investigate how we can mitigate thumbnail poisoning.

How the thumbnail get poisoned:

1. User uploads a link ( A "Cat" for example )
2. Server caches it.
3. User changes it to a banana ( or anything WORSE )
4. Another user sees the cat in the thumbnail, clicks on it and sees a banana instead

Ideas of mitigating it:

Saving the thumbnail hash and let the frontend warn if the hash doesnt match anymore.