Fix EZP-23483: consider PathPrefix if necessary in language switcher redirect

JIRA: https://jira.ez.no/browse/EZP-23483

When switching to/from siteaccess with (different) `PathPrefix` setting, the redirect is often not handled correctly.
This PR considers the current/destination SA's PathPrefix and adjusts destination url accordingly:

* Before matching the url to a node (if it is not a module), check if the current SA uses a path prefix.
If it does, prepend it to url.
* After getting the alias for a node, check if the destination SA has a PathPrefix; if so, attempt to remove it from the redirect (destination) url.

Tests: manual
Add a description to the return value.