cubicweb #3400448 workflow and schema images disappear on first use [validation pending]

As of today, they are wiped on the first render of the src attribute, cubicweb-side.
Calling the src uri a second time (e.g. because one wants to "copy image location" to see it better) yields a logged crash.

They should be e.g. in some appdatahome/uicache subdir and remain there.
Or we build inline svg and the tmppng hack can go away.

prioritynormal
typeenhancement
appeared in<not specified>
done in3.18.0
load0.500
load left0.000
closed by#10888:7b3fa5a7a5a3
patch