Suggest creating alias for `unique_user_email` field validator

[hc-12]

With the introduction of SECURITY_USER_IDENTITY_ATTRIBUTES we can easily add identity attributes (which has not yet been documented).

I can set this in my config: SECURITY_USER_IDENTITY_ATTRIBUTES = ['email', 'username']

With custom form:

from flask_security.forms import RegisterForm, unique_user_email

# an alias to reflect a more accurate usage of the validator
unique_user_attribute = unique_user_email

class ExtendedRegisterForm(RegisterForm):
    username = StringField('Username', [validators.Required(), unique_user_attribute])

tl;dr unique_user_email validator is not quite accurate name with the introduction of SECURITY_USER_IDENTITY_ATTRIBUTES

[jirikuncar]

@hc-12 unique_user_attribute sounds good. Feel free to submit a PR.