REGISTRAR: support for custom validation links

- Allow to use custom email validation link. Base URL is taken from
  new VO or Group attribute (vo as backup for group).

- If provided custom link is not valid (empty) registrar will
  try to use non-authz link as backup value.

- Registrar creates new attributes. After deploy, privileges must be set
  to both: R/W: VO admin and to group attribute also R/W: Group admin.

- Mail manager now doesn't send not valid URLs (empty base url but  with params).
Merge pull request #526 from zlamalp/validation

REGISTRAR: support for custom validation links