cubicweb-squareui #3892039 get rid of nested `container` classes [validation pending]

container class must not be nested

prioritynormal
typebug
done in0.3.4
load0.200
load left0.000
closed by#b773eb12c133 [template] `container` classes must not be nested (closes #3892039)
patch[template] `container` classes must not be nested (closes #3892039) [applied]