[IMP] payment: add new explicit parameters to the payment widget

Purpose of this commit is to ease the use of the payment widget and
avoid having to perform too much custom code in the various routes used
when doing payments :

 * when instantiating the widget give him its parent element data so
   that parameters given when calling the payment form template are
   automatically present in the widget options;
 * support more parameters when calling the route for form-based acquirers
   including access_token, URLs and callback method;
 * when doing a form-based payment a call to a JSON route returning the
   rendered form is done. Payment widget now uses all data from that
   form including the URL of the acquirer website. This way form-based
   acquirers are really dynamic and values update is easier;
 * remove hidden display of form-based acquirers as all data come now
   from the called JSON route that returns the form;
 * add a warning about partner-id not being set as it is an issue some
   people may encounter;

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