REGISTRAR, GUI: support for inviting users to VO/group

- New application notification type "user_invite".
- Allow to invite unknown (by email) or known (by user) persons into VO/group.
- Link to VO's/group's application form is sent, based on current membership
  status of known user. For unknown, link with VO->Group is sent, so they can apply to both at once.
- Notification language is determined from User/VO/Group preferences or set
  manually, if user is unknown.
- If anything fails (template not defined, user is member,...) notification is not sent
  and vo/group admin is notified in GUI.