Search for logged in users on the Users Overview page

## What?

As a Graylog admin I occasionally need/want to identify all users currently logged in to an environment. My current workflow is to hit the `Users Overview` page (`/system/users`) and run through the `Users` list. In an environment with a large number of users this process can be a bit tedious.

This is a request to expand the search syntax for the `Users` section, adding support for filtering on logged in status (e.g. `logged_in:true`). For bonus points, other columns (Client Address, Enabled, Role) might also be nice.

## Why?

Sometimes I simply want to know who is using a particular environment. Occasionally I need to reach out to logged-in users ahead of off-hours maintenance.

Being able to search on the other columns might also be useful in certain situations (e.g. finding all disabled user accounts, users logged in (or _not_ logged in) from a certain CIDR prefix, etc.).

This could be scripted easily enough, but it seems like a need others might also have.

## Your Environment

* Graylog Version: 4.1.6