cubicweb #3471609 3.18 migration is not paranoid enough [validation pending]

things explode if an attribute is in the db but not in the fs schema, e.g. if a drop_attribute was missed in some earlier migration.

prioritynormal
typebug
done in3.18.3
load0.250
load left0.000
closed by#07fc991612b0 [migration/3.18] protect against attributes in db missing from fsschema
patch[migration/3.18] protect against attributes in db missing from fsschema [applied]