Show delete date and name of deleter in list of deleted users

In the user management deleted users are listed. For reasons of transparency it is important to see who deleted someone when. The following is implemented: 

• Add a field deletedDate to the identity object
  • date field populated by delete process
• Add a field deletedBy to the identity object
  • text field populated by the delete process
    • Firstname and name of user who initiated the deletion process if the deletion process was not initiated by user himself
    • "Account deleted by user" if deletion was initiated by user himself
• Add a field deletedRoles to the identity object
  • text field populated with the name of the roles the user das before deletion (usermanager, administrator...)
• The list shows the rows username, firstname, lastname, deleted date, last login date, created date, deletedby, deletedRoles and an action link to clear firstname, lastname.

The clear feature is necessary to clear firstname and lastname of administrative users who's name will be kept during deletion, see OO-3476