[FIX] web: do not call session load_qweb on frontend

Since commits 2a8a17024d and 9c68bc7482,
the session is not started by the webclient anymore but directly by the
session service instead, and the session info is found in the DOM
instead of by RPC.

Because of that, the session is started in the frontend and other
non-backend environments which caused a JS error while trying to
load qweb (as no qweb has to be loaded). This commit makes the
session start check if it is a non-backend environment and only
initialize session info (from DOM) in that case.

addons/web/static/src/js/framework/session.js

@@ -86,8 +86,11 @@ var Session = core.Class.extend(mixins.EventDispatcherMixin, {
      * Init a session, reloads from cookie, if it exists
      */
     session_init: function () {
+        var def = this.session_reload();
+        if (this.is_frontend) return def;
+
         var self = this;
-        return this.session_reload().then(function() {
+        return def.then(function() {
             var modules = self.module_list.join(',');
             var deferred = self.load_qweb(modules);
             if(self.session_is_valid()) {