Fix EZP-21354: Siteaccess part missing in generated symfony routes

https://jira.ez.no/browse/EZP-21354

Link generation against standard (declared) Symfony routes didn't prepend siteaccess when needed, hence forcing the default siteaccess, which could cause further issues in custom controllers.

This PR fixes this issue and also adds an *excluded routes* setting, to be able to declare which routes are **not** supposed to be siteaccess-aware (e.g. Assetic routes).
One can add excluded routes with following semantic config (master only):

```yaml
ezpublish:
    router:
        default_router:
            non_siteaccess_aware_routes: ['foo_route', 'my_route_prefix_']
```

Tests are in a separate commit, to ease backport.