prevent loading render.js and rsvp.js twice from different url

@romain /cc @cedric.leninivin

Patch for spreadsheet gadget works in classic erp5 standard ui too and for @jerome gadget works in renderjs_ui as well as in erp5 standard ui.

https://lab.nexedi.com/bk/erp5/blob/master/bt5/erp5_graph_editor/SkinTemplateItem/portal_skins/erp5_graph_editor/dream_graph_editor/jsplumb/index.html.xml#L32