slapos_jio: Fix 'Go Back' link on login_form

See merge request nexedi/slapos.core!382

master/bt5/slapos_jio/SkinTemplateItem/portal_skins/slapos_hal_json_style/login_form.zpt

@@ -44,7 +44,7 @@
           <section tal:condition="not: portal/portal_membership/isAnonymousUser">
               <p i18n:domain="ui" i18n:translate="" >It seems you're already authenticated.</p>
               <p><a tal:condition="exists: request/came_from"
-                     tal:attributes="href request/came_from" >Go back</a></p>
+                     tal:attributes="href python:context.WebSection_renderCameFromURITemplate(request.came_from)" >Go back</a></p>
           </section>
           <section tal:condition="portal/portal_membership/isAnonymousUser">
             <section class="ui-content-header-plain">