Add support for linking to external users in entity search

data/users/database.py

@@ -1,6 +1,10 @@
 from data import model
 
 class DatabaseUsers(object):
+  @property
+  def federated_service(self):
+    return None
+
   def verify_credentials(self, username_or_email, password):
     """ Simply delegate to the model implementation. """
     result = model.user.verify_user(username_or_email, password)
@@ -16,3 +20,11 @@ class DatabaseUsers(object):
   def confirm_existing_user(self, username, password):
     return self.verify_credentials(username, password)
 
+  def link_user(self, username_or_email):
+    """ Never used since all users being added are already, by definition, in the database. """
+    return (None, 'Unsupported for this authentication system')
+
+  def query_users(self, query, limit):
+    """ No need to implement, as we already query for users directly in the database. """
+    return (None, '', '')
+