Graylog administrator should be able to register and configure Graylog users who are registered in Active Directory

## Expected Behavior
Graylog administrator should be able to register and configure Graylog users who are authenticating using Active Directory credentials, both in the Web interface, and using the REST API, before the user authenticates for the first time.
Therefore, it should be possible to register a user without a password if this user is authenticating using Active Directory. Because there in no password in a user's JSON if this user has been registered after an authentication using Active Directory.
It would also be useful to register in the user's JSON that this user is authenticating using Active Directory.

## Current Behavior
The password is mandatory when the administrator tries to register a new user in the Web interface (Authentication > Users > Add new user), or using POST /users in the REST API Browser.
The only way to register and configure a user who is authenticating using Active Directory is using this procedure:

1. The user authenticates in the Web interface using his Active Directory credentials. At that moment, his account is not configured. For example, he has no additional role if the roles are managed by Graylog administrator (i. e. not using LDAP groups).
2. The administrator edits this user in the Authentication > Users page, and configures his account
3. The user reloads the Graylog page to access the resources

## Possible Solution

## Steps to Reproduce (for bugs)

1. The administrator configures an Active Directory authentication
2. The administrator tries to configure a user without a password

## Context

## Your Environment

* Graylog Version: 2.4.6+ceaa7e4, codename Wildwuchs
* JVM: Oracle Corporation 1.8.0_172 on Linux 3.13.0-153-generic
* Elasticsearch Version: Version: 5.6.3 (provided with Graylog AMI)
* MongoDB Version: v3.4.9 (provided with Graylog AMI)
* Operating System: Ubuntu on AWS (Linux graylog 3.13.0-153-generic #203-Ubuntu SMP Thu Jun 14 08:52:28 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux)
* Browser version: Firefox, on a Windows PC in a business environment (proxy + firewall + security tools ...)