[Feat] Using Unsplash when Lexica is down

[sebbourgeois]

Lexica is down pretty often these days, and the support is really slow to answer, it usually takes at least a week to get the service back online.
Can we use Unsplash instead of Lexica when it is down ? It might not be ideal but it would be better than the current black square we get ?
We can get a random image at this url : https://source.unsplash.com/random/1280x720
Or we can configure the bot with an API key so we can something more accurate (https://unsplash.com/documentation).