username_unique
Check if an entered username is unique by querying model#identity for the given
username:
<input type="text" id="username" name="username" value="" />
{% validate id="username" type={username_unique} %}
Optionally, an id parameter can be given to the validator to skip that particular id when doing the uniqueness check.
This is useful when you are displaying a form in which the user is editing his own user name.