cubicweb #2110264 Unify JSonController._call_view and TheMainTemplate.call [open]

JSonController._call_view implementation is a nearly complete duplication of TheMainTemplate.call(). Since the first one tries to emulate the second one, unify both or make the former use the latter.

prioritynormal
typeenhancement
done in<not specified>
load0.100
load left0.100
closed by<not specified>