* add checkbox "substituable" to admin/subdef ; bump to 4.1.8-rc7 and bump production-client ; migrate and remove conf 'registry/modules/thumb-substitution'
* bump production-client to 94
* fix test ; add test ; move "substituable" node to all subdefs (attribute)
* fix test
* fix test
* set "flatten layers"=true for new subdefs (PHRAS-3852 fix it)