[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.

Latest changes 
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* table of `all latest changes`_

Links below is providing useful RQL query example.

.. _all latest changes: view?rql=Any+M%2CX+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30