fix: reintroduce lost NoDuplicateHook lost while losing 1.9.1 code

Apparently this code was lost by being (maybe) pushed on hg.logilab.fr and
never pushed to our new forge.

cubicweb_link/hooks.py

+from cubicweb import ValidationError
 from cubicweb.predicates import is_instance
 from cubicweb.sobjects.notification import ContentAddedView
@@ -1,8 +2,9 @@
 from cubicweb.predicates import is_instance
 from cubicweb.sobjects.notification import ContentAddedView
+from cubicweb.server.hook import Hook
 
 
 class LinkAddedView(ContentAddedView):
     """get notified from new links"""
     __select__ = is_instance('Link')
     content_attr = 'description'
@@ -3,6 +5,26 @@
 
 
 class LinkAddedView(ContentAddedView):
     """get notified from new links"""
     __select__ = is_instance('Link')
     content_attr = 'description'
+
+
+class NoDuplicateHook(Hook):
+    """ Before adding/updating a link, check that the URL is not already in the
+    database.
+    """
+
+    __regid__ = "link_no_duplicate_hook"
+    __select_ = Hook.__select__ & is_instance("Link")
+    events = ("before_add_entity", "before_update_entity")
+
+    def __call__(self):
+        try:
+            new_url = self.entity.cw_edited["url"]
+        except KeyError:
+            return
+
+        if self._cw.find("Link", url=new_url):
+            msg = self._cw._("This URL is already existing")
+            raise ValidationError(self.entity.eid, {"url": msg})