Fix EZP-22735: Make {{ path( 'ez_urlalias' ) }} polymorphic

A fix for already existing issue at JIRA: https://jira.ez.no/browse/EZP-22735

Common usecase is to generate an URL to content in Relation field value.
Let's use `ContentInfo` only. I'm really not comfortable with `$content` being polymorphic here. As @andrerom said, `ContentInfo` is always available.