Fix EZP-21758: Custom variables are not correctly exposed in legacy templates when doing a sub-request

[EZP-21758](https://jira.ez.no/browse/EZP-21758)

See issue description. Basically, user parameters need to be exposed as regular variables in legacy templates.
Please note that user parameters can only be passed by calling eZModule directly, which is done in ezpublish-kernel for [legacy LocationViewProvider](https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/MVC/Legacy/View/Provider/Location.php#L77).

This is a blocker for DemoBundle, so should be in 5.2.