cubicweb #6893185 uicache mechanism gets easily confused [done]

Once a file has been generated in uicache, we assume it'll stick around forever, which means if a file in uicache is somehow deleted, we'll return 404 until we're restarted.

One option to fix this

prioritynormal
typebug
done in<not specified>
load0.500
load left0.000
closed by#7c386161ebd6 [web] remove PropertySheet._cache
patch[web] remove PropertySheet._cache [applied]