Fixed one typo in response_helpers which caused is_admin context variable not to work correctly. Created new checkLinkNameForId function that checks if link_name is correct when creating new user and modifying existing one. This is now used for validation instead of doesLinkNameBelongToId in UserForm clean_link_name function. The previous validation function didn't allow to create new users and modify linkname of existing ones. Additionally I created new doesLinkNameExist helper function.

"""
Field-like classes that aren't really fields. It's easier to use objects that
have the same attributes as fields sometimes (avoids a lot of special casing).
"""

from django.db.models import fields

class OrderWrt(fields.IntegerField):
    """
    A proxy for the _order database field that is used when
    Meta.order_with_respect_to is specified.
    """
    name = '_order'
    attname = '_order'
    column = '_order'