admin,storage: added more administrator functionalities

administrators can now:
 - Set displayname of users
 - Update user avatars
 - Search for users by user_id
 - Browse all users in a paginated API
 - Reset user passwords
 - Deactivate users

Helpers for doing paginated queries has also been added to storage

Signed-off-by: Morteza Araby <morteza.araby@ericsson.com>