Adjust hub_edit_post behavior for request made from javascript
    
    If a POST request is made from javascript, we do not want to flash
    messages, as they will appear the next time someone load a page,
    potentially a completely different page.
    So, we will assume request made from JS will declare themselves as such
    using js=true and for them we will make things easier: work (200 OK) or
    does not work (400), no message flashed.