fixed bugs and inconsistent behaviors in user management

- groups and groups permissions are now always prefixed with one @ (automatically on creation)
- users & groups are really deleted (not only permissions)
- illegal characters are removed from user/group names
- user is created on blank password (not only permission)
- can't create user/group name with length < 3
- increased default size of user/groups queries in elasticsearch to allow more than 10 users & groups