[ajax] refactor/cleanup low-level ajax functions * loadxhtml/replacePageChunck/reload_component/reload_box deprecated in favor of loadXHTML / ajaxFuncArgs * some other cleanups in cubicweb.ajax.js * add js_render which will replace js_component (more generic, nicer argument passing handling)

[querier] test and fix potential bug in insertion query with multiple relations inside, caused by a name conflict. Also fix another bug non reproduced by using a set instead of a list (list may leads to compare entity instances which isn't allowed).

[form] avoid spurious warning subsequent for form's action refactoring w/ autoforms

backport stable