New user state "pending" for not-yet-activated users

Currently we have user status "active", "active and undeletable", "login denied" and "deleted". 

Specially in case of enabled self-registration a "pending" user status would be very helpful. 

Pending users...

• Are listed in the user management in a pending users list
• Can not log in
• Can already have a password (but don't need to)
• Can not be looked up in normal user searches (like deleted users)

Todo:

• Implement data model for pending users
• Implement managers, services, queries, REST etc. 
• Extend user status edit form in user management user details and add new status
• Extend user search form in user management and add new status
• Implement list of pending users similar to the list of blocked users
• In user change bulk wizard add new status

The new mail config should consider the existing config, namely registration.enableNotificationEmail and registration.notificationEmail and make them configurable in the UI as well.