[js] make _loadAjaxHtmlHead aware of mod_concat-like urls _loadAjaxHtmlHead keeps track of already imported resources (js / css) to make sure they're not included twice, but with mod_concat-like urls, things get a bit more complicated : if BASE_URL/data??foo.js,bar.js is already imported, then loading : BASE_URL/data??bar.js,spam.js should only load spam.js

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