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:

* `destinationUrl()` is called when switching from current siteaccess A to B
* If the current siteaccess A has a PathPrefix ('SomeRoot'), the uri '/content' is in reality '/SomeRoot/content'
  This needs to be taken into account when fetching the corresponding nodeId by path string

* If the siteaccess B has a PathPrefix ('SomeOtherRoot'), the destination url must also take this into account, and exclude the element(s) from the uri.
* However, if 'SomeOtherRoot' is not present in the URL but a pathPrefix does exist, 
  the PathPrefix from the current siteaccess ('SomeRoot') is excluded instead.

  This can happen when switching between the same node with multiple translations/prefixes.

Tests: manual
How can this not be a string? And this is not documented in PHPDoc.