Admin API improvements

[davidmz]

The GONE_SUSPENDED user status now doesn't allow user to activate her account back.

Added

• New Admin API methods:
  • GET /api/admin/users method returns all users sorted by registration date.
  • GET /api/admin/users/:username/info method returns information about the specific user.
  • POST /users/:username/suspend and POST /users/:username/unsuspend methods suspended/unsuspended given user.

Changed

• The POST /api/admin/users/:username/freeze methods now accepts 'freezeUntil' parameter in the following formats:
  • ISO Datetime
  • ISO Date
  • ISO Duration ("P...")
  • The "Infinity" string (means forever freeze)
• The admin user serializer now returns 'freezeUntil' and 'goneStatus' fields.