[ajax] improve backward compatibility for redefined ajax functions (closes #2447189) If an ajax function from stdlib (e.g. js_reledit_form) was redefined in a cube, the cube version should still takes the precedence< => check for existing ajax function on deprecated controller _before_ checking ajax function availability on the new one.

doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc