cubicweb-file #2886070 CACHE the thumbnails instead of recomputing them on the fly all the time [validation pending]

It's a bit uncertain we really want to systematically recompute them.

Will also provide a new protocol/adapter for thumbnails.

priorityimportant
typeenhancement
done in1.15.0
load1.000
load left0.000
closed by#1a4ef257e5d3 full thumbnail support for images (closes #2886070)
patchfull thumbnail support for images [applied]